What Gothmog said.
Gogf, you are constructing a strawman; if you want to define agnostic as someone who requires "absolute proof" of everything, then go ahead, but the discussion will be useless as no one is agnostic by that definition. You might as well argue against Christianity by defining Christianity as the belief that a giant seahorse named Jesus became pregnant despite being female, even though male seahorses are the ones that have the babies. Yes, it would be rather easy to attack "Christianity" in that case, but why bother?
I know you said not to do this, but I feel I should give an alternative definition of agnosticism: There are two camps, the "hard agnostics" who believe that it is fundamentally impossible for humans to acquire knowledge regarding the existence or nonexistence of God, and the "soft agnostics" who believe it might be possible to do so but who also believe it has not been done yet. I consider myself a soft agnostic (and a soft atheist, which pretty much all soft agnostics are---both deal with the idea I should just tentatively assume that God does not exist, considering the lack of evidence, but that it is possible that he does exist).
But anyway, enough about agnosticism. I think your ideas, Gogf, are interesting when taken as general philosophical inquiry, rather than a specific attack on agnosticism. Like Gothmog said, you're dealing with the idea of a priori principles or a "first philosophy." You also are dealing with the notion of "proof," and the basic nature of science.
We all have our experiences, and then we have thoughts to make sense of these experiences. Many (probably most) philosophers have suggested that the latter must come first; that is, we must have some sort of "first philosophy" that we can add our experiences onto. For example, everyone (except for maybe the occasional wacko) operates under the assumption that our five senses should, in general, be trusted. Your mother exists, because you can see her. If you smell smoke, you should run, as there's a fire. The alternative would be to assume that our five senses should NOT be taken as good ways of getting information, and we should ignore them. (Obviously that would be crazy by most people's standards.) Whatever the case, you are starting with a "first philosophy" that you build everything else onto. Doing otherwise (building something out of nothing) is as logically impossible as drawing a square circle. The assumptions that you use to make your first philosophy are
a priori principles ("a priori" = "before experience"). The scientific method is an example of a first philosophy.
Again, I disagree with your characterization of agnostics, but for the sake of argument, even if agnostics do indeed demand full proof of something in order to believe it, this is not an inconsistent philosophy; rather, it is just a case of applying a first philosophy.
I think you are mischaracterizing science and "proof." "Proving" something means showing that it logically follows from one or more given statements. Of course, those given statements can themselves be proven by showing that they in turn follow from even more fundamental given statements, which can in turn be proven by ... etc. When using this process, obviously you need to start somewhere: with a priori principles, or axioms. The axioms themselves cannot be proven, as there are no more fundamental statements to work with. Contrary to what you said, proof is impossible in science, and scientists make a living by constantly proving things
wrong---no theory is ever proven right, but instead can only be proven wrong, and when a theory has been tested and tested over and over again and has never been shown to be wrong, we become more and more confident that it's right, but never 100% sure. "Proof" is something only found in mathematics and logic, for obvious reasons. And I'm sure most agnostics understand this.
Some philosophers disagree with the above idea that we must have a "first philosophy," and instead they think we can jump right into our experiences without having any sort of epistemological principle to start with. The most important of these philosophers would be
W.V. Quine. I'm not sure what to think about that, though.