You're (all) right, of course, there are several mathematicians who refuse to have an opinion on whether P = NP, and a handful who take the minority view (although of the 8 who did so in Gasarch's survey 'some' admitted they were doing it just to be contrary, that really doesn't leave many who actually believed P=NP).

What this definitively does not mean is that it's rational to assign 50% probability to each side my main point was that there is ample evidence to suggest that P != NP (see the Scott Aaronson post I linked to above) and a strong consensus in the community that P!=NP. To insist that one should assign 50% of one's probability to the possibility that P=NP is just plain wrong. If nothing else, Aaronson's "self-referential" argument should be enough to convince most people here that P is probably a strict subset of NP.