In this presentation (which mutated into a debate) one certainly gets the sense that Alvin Plantinga is just plain bluffing. He throws up plenty of nifty maths onto the whiteboard, but these serve primarily to obfuscate his false premises rather than bring enlightenment to the audience. Plantinga’s evolutionary argument against naturalism was thoroughly rebutted some time ago, and he seems even not to be unaware of these cogent criticisms of his position. Moreover, he seemed not to notice when Dennett explicitly rejected and refuted his key premise.
A bit of background is necessary here, because Plantinga's argument is fairly technical and most people don't much grok maths, especially Bayesian probabilities. Suffice to say that for his argument to go through he must show that humans most always form true beliefs about the world [ P(R)≈1 ] and that probability of this happening if metaphysical naturalism and evolutionism are true is low [P(R|E&N)≈0].
Alas, Plantinga fails to substantiate either of these claims in anything like a rigorous logical fashion. He more or less assumes the truth of the former premise and pretty much hand-waves his way to the latter. When a brilliant logician such as Alvin Plantinga is waving his hands instead of outlining a deductive argument, well, caveat emptor.
A couple points must be made here. Metaphysical materialists cannot assume P(R)≈1 since we believe that all talk of gods, spirits, ghosts, chakras, etc. is all so much bunk. People around the world make up all sorts of wacky beliefs about disembodied minds and imaginary forces emanating therefrom, thus, P(R) is evidently nowhere near unity. Moreover, since most religions (with a few interesting exceptions) assert that all other religions make up all sorts of untruths about the world, which are integrated into their devotees worldviews, it seems odd for any religious person to argue that humans almost always form true beliefs about the world. Finally, it should be evident from the abundance of material at sites like http://www.snopes.com/ and http://www.nizkor.org/features/fallacies/ that we humans are prone to all manner of irrational thinking, not least of which is a tendency to attribute agency where none exists.
Secondly, while the probability P(R|E&N) is nowhere near unity, it is neither so low as to allow Plantinga's argument to go through. The crucial question here is whether we would expect naturalistic evolutionary mechanisms to select for true beliefs over false ones. This question is not nearly so simple as it sounds (or as Plantinga's treatment suggests) but it should be fairly obvious that it is generally far easier to program a neural network to solve problems of adaptivity by providing adaptive goals and good data than by providing maladaptive goals and bad data.
Dennett managed to raise some of these points by way of an awkward analogy, but to be fair he was dealing with a mathematical smokescreen while standing up. This is something no one should be expected to do.