Interestingly enough, according to some Muslims, he is, indeed Muslim. A Hotel California kind of thing: You can check out anytime you like, but you can never leave. His father was born a Muslim and apparently left the religion at some point.
By any reasonable measure, Sen. Barack Hussein Obama, Jr. was never Muslim, but Islam isn't known for its reason. Some Imam somewhere will issue a fatwa against him.
Which will be good for a few million votes.