According to various polls, between 20 to 24% of Americans incorrectly believe that Barack Obama is a Muslim. The White House describes Obama as a "devout Christian" who prays every day.But Jerry Coyne and Bill Maher argue that Obama is an atheist.
As mentioned before, there are reasons for saying that Obama is a Moslem. It is customary to call someone a Moslem if he was born a Moslem and does not overtly repudiate the faith. Yes, Obama says he joined Rev. Wright's church and wrote about it in his autobiography, but he also quit the church when he found out what was going on there from public press reports. Apparently he never attended enough to find out for himself.
So I do not think that it is a misconception just because people tell a pollster something different from what the White House spokesman says.