CNN's Candy Crowley says people think President Obama is Muslim because he doesn't go to church regularly, like George Bush did:

The lack of any kind of evidence to the contrary, and by that I mean, the pre-- George Bush went to church, talked about Jesus Christ and God fairly regularly as you know. This president doesn't tend to do that. … I think we don't see him going to church, and so that may somehow stir up some thought like 'I don't really know what he is.'

And according to the AP, some "Pew analysts" agree:

Pew analysts attribute the findings to attacks by his opponents and Obama's limited attendance at religious services, particularly in contrast with Presidents George W. Bush and Bill Clinton, whose worship was more public.

Where did people get the idea that George W. Bush regularly attended church? He didn't. Whatever the reason(s) for the widespread false belief that President Obama is Muslim, they aren't so innocuous as his infrequent church attendance.

Last year, some reporters wondered why Media Matters criticized them for reporting on the infrequency of Obama's church attendance without noting Bush's infrequent attendance. Maybe now they understand?