It's not as if there aren't many, many great physicists who have failed to understand quantum mechanics. Max Planck invented the field but never understood it. And Einstein, if he did indeed understand it, never stopped wishing that he didn't have to.
Anyway, I haven't read Penrose's arguments -- geez, I barely take time to read credible science these days -- so I'm not dismissing him: I'm outsourcing my dismissal of him to guys like Dennett. Meanwhile, I'll happily accept your assertion that Penrose understands quantum mechanics perfectly well even if he enjoys abusing it for philosophical kicks, kind of like how the screenwriters who wrote Gladiator claim to have actually read real Roman history before they reinvented it for the screen.
From now on I'm outsourcing all my dismissiveness to Daniel Dennett. I just don't have the time to be properly dismissive, and he's so much better at it than I am anyway.
I also think he's wrong, but at least he's not infatuated about it.
edit: I love his books. Well worth the read, and for those concerned he puts his speculations in chapters very clearly marked "speculations". From his books I first learned the details about Turing machines and lambda calculus, and a lot more.
It all hangs, not on quantum mechanics, but on his initial assertion. I've read his explanation in Shadows of the Mind, but the lightbulb didn't go on for me. For McArthy to dismiss the question offhand was a little disappointing. I'd expect him to have a deeper insight.
What the quantum intelligence does is take one unresolved problem (how do we think) and replace it with another (we think with quantum computing, but we don't know exactly how). Its only result is to take away the unknown and replace it with an incomplete explanation.
I also don't think Penrose would have come to this conclusion now. Cognitive psychology has taken enormous strides in the past years, and it's already pretty clear it's on the right track. Take a look for example at "The Emotion Machine" by Marvin Minsky (who by the way is on par with McCarthy in AI but chose cognitive psychology as a main field: http://en.wikipedia.org/wiki/Marvin_Minsky).
[1] Spivey, M., Grosjean, M. & Knoblich, G. (2005). Continuous attraction toward phonological competitors. Proceedings of the National Academy of Sciences, 102(29), 10393-10398.
Q. Is there anything in principle that would prevent a computer from thinking as a human would?
A. No
IOW, there's still no recognition today, on the side of the purveyors of "classical" AI, that anything except digital processing might be needed for a computer to think "as a human would". So the big money is likely to continue being thrown at attempts to emulate animal brains using purely digital means. And I suspect that these funds might largely be better spent elsewhere.