Gallup headlines this:
Many Americans Can't Name Obama's Religion
Majority guess he is Christian, 11% say Muslim
However! The question was open ended rather than multiple choice. I would hope that if asked, I would have gone with "Protestant" or "Christian". However, if my Inner Pedant were responding, "Don't Know" might have come up since I have no idea what Protestant denomination Obama might be associating himself with these days, or ever.
So it strikes me as plausible that "Don't Know" includes a lot of folks who would have picked "Protestant" if prompted.
AND BTW: Did the pollsters provide a a breakdown between "Sunni", "Shi'ite", and "Other Muslim"?