Nowhere have I ever seen criticism of what should be the very central informational construct of the SETI (Search for Extraterrestrial Intelligence) effort.  Officers of the SETI Institute and others never give you a straight answer on this issue when you pin them down on it.  Let the matter be laid out and discussed here.
 

INTRODUCTION

I'm sure there are many others better qualified than I to present this discussion, but I've never heard anyone else raise this issue, at least in the manner I wish to, so here goes.  The issue here is not directly that of whether or not extraterrestrial "intelligences" exist, however one wants to define them, although any decision on the issue here would likely cause one to reconsidering how such entities should be defined.  For the moment, let us assume such things exist in some form that would be generally recognized as intelligences, either immediately or after extended scrutiny, should they appear on your front lawn.  A prominent zoologist, Ernst Mayr, states his biological rationale here as to why such entities, conceived as carbon-based organisms, likely don't exist elsewhere in the universe.  Dr. Mayr blames the fantasy of ETIs on the fact that most of the searchers are "astronomers, physicists and engineers," and that they are "simply unaware" that the success of SETI depends, not on physical laws, but on "biological and social factors."  I don't know about that, but what I, who have an engineering background, discuss here doesn't involve biological or social thinking at all; and I consider the basic mathematical-informational argument here to be much more fundamentally threatening to SETI, than such thoughts of life and living as we know it.
 

VERBAL DESCRIPTION OF A HIT

I have seen, in the past, elaborate formalizations of when and how SETI people would announce to the general public, or to some more restricted group, the verified reception by them of whatever sort of "signal" they feel constitutes that having come from an "intelligence" (or whatever lesser entity they are momentarily settling for).  Apart from those who simply accept the word of the gods of science as unresolvable TRUTH on the face of it, one would presume the public would expect two descriptions in such announcements from successful searchers -- 1) the nature of the signal and how it can be concluded that it came from an "intelligence", and 2) the nature of that "intelligence".  The SETI boys and girls could probably successfully argue against having to extrapolate an answer to '2)' right away, but '1)' drives to the essence of what they keep trying to sell the public (and more especially to those with too much cash around in the vicinity of a gullible curiosity).

It appears to me that, although the scientists and engineers associated with the SETI Institute are quite sophisticated in their thinking at levels of physical science and radio-electronic and optical engineering, like many other scientists, engineers and other sorts of specialists, they can retain a very unsophisticated view of a crucial element in the exciting adventurous endeavor they decide to hop into.  It appears elsewhere on the Web that prime numbers constituted the signal sent by the aliens in the movie Contact, as a signal that could not arise by any "natural" phenomenon of which we on earth are aware.  It is my impression that the SETI people generally are soaring on an intuitional feeling of what is natural versus what is artificial, in the simple way anyone would be apt to naively make this distinction as to very earthly matters, in cases where this mode of distinction would normally be adequate to one's application of it.  Physical science, and the mathematics that makes it work, just don't define the concepts 'natural' and 'artificial'.  Physics and science on up through biology take a quite un-observer-centric view of the world, a view that fully holds that people, including the observer, are part of the natural world, the only world these disciplines recognize.
 

MATHEMATICAL DESCRIPTION OF A HIT

I think the only concepts even a virtual traveler around the universe can hang onto are complexity and degree of uniqueness, neither of which measures care a hoot for the anthropocentric distinction natural / artificial.  Things have a particular relative order of complexity based on recognized common components in their makeups.  Of course, we can calculate the order of complexity for the abstract mathematical production of a prime number of any order and also the order of any minimal mechanism that would be required to do same, relative to other mechanisms built of the same components, and even pretty good estimates of the relative order of earthly organisms that would have any given probability of doing same -- whether they looked like us or said they were of the "natural" or of the "artificial" persuasion.

I suppose we can assume we can decipher how any gismo, "artificial" or "natural", and of any order of complexity, represents prime numbers, Fibonacci numbers, or whatever else be chosen as the inter-stellar-system handshake.  Evolutionists might claim such number sequences have no survival value and that only "intelligences" could produce them -- as unavoidable artifacts; but certainly, an inadvertent logic-network, formed as a byproduct in an organism, could evolve for at least a limited time -- the same as with the correlate human geek pastime that has occurred in certain human intelligences.

But at a much less sophisticated level, decades ago when I spoke with Jill Tarter at a Mensa meeting, and a couple years ago when I inquired of Frank Drake at an IEEE meeting -- in regard to the necessary criteria for "artificial" signals -- these people tended to back down very far from having to receive something that could be interpreted as a significant set of prime numbers in order to announce their success to the world.  In fact, Dr. Drake seemed to be content with just some signal that looked more "regular" than what has been seen from space to date -- supposedly just a little more regular than those from pulsars or whistlers.  And, as I recall, Dr. Tarter would slip down from "intelligence" to simply "life".  Well, of course, the line between an intelligence and a live form without it is rather arbitrary -- except in respect to the concern, in earthly terms, of what sort of matter could modulate sufficient power to emit signals receivable on earth.  And today the line between life and inanimate matter is pretty blurry at the level of various sorts of viruses and separable subprocesses of organisms.  But, whatever, the 'I' in SETI doesn't exactly stand for 'life', which, in English, begins with 'L'; so I won't SETL for that.  (OK, granted that life that does that with symbology ought probably to be removed from earth.)
 

SUFFICIENT COMPLEXITY WITHOUT A CARBON BASIS

Although SETI searches the sky for "intelligent" signals out to really considerable distances, it appears to conceive of such signals as being possible only under control by the sort of carbon-based (living) organisms we find controlling such emission of signals here on earth.  Must we assume only such critters would have to be at a targeted location in space in order to get whatever degree of improbability of whatever level of complexity of signal SETI should receive and announce as one of "them"?  Could we not consider extraterrestrial evolution to have produced something like an organism that would have, built into its anatomy, an organ that could transmit sufficient radio or light energy to reach earth?  Could we not consider something very unlike a carbon-based organism, say a rather organized gas plasma, that could do the same?  Call these sorts of things 'alive' or 'dead', 'natural' or 'unnatural'; I don't care -- and you and the problem space shouldn't either.
 

TO WHOM OR WHAT AM I SPEAKING?

Consider that such a "recognition" channel is extremely narrow as a tool for observing what sent whatever be interpreted as a signal from a local inhabitant.  What kind of tiger can SETI possibly say it has by the tail?  I'm not sure that that first public announcement is going to communicate much of anything reasonably accurately.  I think, in general, one can say that, as one attempts to verbally describe things at extreme distances over either time or space, all common concepts approach complete uselessness/accuracy.
 

SKEPTICISM

Last month I went to a meeting of the Bay Area Skeptics, where David Anderson and Dan Werthimer spoke on SETI@home.  In response to a question along these lines, that I put to them, I got only the "I'm not sure I understand" approach and a lot of beside-the-point positive fill-in on SETI.  Afterwards, a BAS officer asked me if I thought there was something improper (I can't remember the exact word used) about SETI.  I certainly wasn't taking any moral stance against SETI.  As far as I know the US government isn't supporting any of it at this point and I doubt any foreign governments are.  SETI does appear to be presently further infiltrating the UC system of the State of California somewhat, in the form of Serendip IV, but I'm not railing on the politics of its financing and certainly don't deny private parties the right to support such an effort, whether or not they understand it in any sense.  But the stance at BAS didn't seem to be skeptical at all; the atmosphere seemed to regard it as mom or apple pie, and thus above criticism.  I thought skepticism critiqued pretty much anything that attempted to expand the current limits of science or claimed to apply science in an unusual way.  In any case, I feel SETI is open to the sort of criticism I haven't done a very good job of in this presentation -- simply on an intellectual basis.  Hey, science has long accommodated to that sort of thing; it's not exactly religion, after all.