Second, it's not the far right, or even the right, that's presuming that Obama is a Muslim. The Slate article was in response to a PPP poll. PPP is a Democratic polling outfit, and they've cooked previous polls to give results that they wanted to push.
Third, Quite a few independents and Democrats also think that he's a Muslim. The reasons for this have little to do with his race (although his middle name probably doesn't help), but has a great deal to do with his policies (this is the man who redefined NASA's core mission as Muslim outreach, after all).