According to this harticle in today's NY Daily News, 20% or more (depending on the poll) of Americans "wrongly believe" that President Barack Obama is a Muslim, and the number has grown since Obama asserted (correctly) that the Islamic Community Center has the right to locate near "Ground Zero" in southern Manhattan. Even before he was elected president, fringe elements on the right were alleging that he was a "secret Muslim" (see, e.g., here), but 20% is too big to be called a "fringe."
I actually hope the people who claim to "believe" Obama is Muslim are purposely lying about their own beliefs for political purposes because if they actually believe what they say they believe, contrary to all the evidence, the implications would be very troubling. Is it really so easy for us to pull the wool over their own eyes and convince ourselves of established untruths? What else are individuals willfully deluding themselves about, contrary to the facts, in their daily lives? Can the rationality assumption of economics and positive political theory stand, if self-delusion is a fundamental part of our make-up?
Perhaps I am overreacting about the implications. This is a comparatively easy case for self-delusion because, after all, one's religious commitments are always privately held, whatever one might say about them in public. However well-established is Obama's Christianity in the public record, questions can always be raised about his, or anyone else's, private beliefs. For example, if we believed that Pat Robertson is a secret Satan worshiper, we could explain away his apparent commitment to Christianity as nothing more than subterfuge.
The early 20th century writer and social reformer Upton Sinclair famously and sagely wrote, "It is difficult to get a man to understanding something when his salary depends on his not understanding it." Perhaps it is also difficult to get someone to understand something when her political commitments depend on her not understanding it. Either way, I'd still like to believe that facts matter.