As the son of the Muslim father, Senator Obama was born a Muslim under Muslim law as it is universally understood. It makes no difference that, as Senator Obama has written, his father said he renounced his religion. Likewise, under Muslim law based on the Koran his mother's Christian background is irrelevant.Not something that I'd seen explained before. Certainly it could complicate things... but...
His conversion, however, was a crime in Muslim eyes; it is "irtidad" or "ridda," usually translated from the Arabic as "apostasy," but with connotations of rebellion and treason. Indeed, it is the worst of all crimes that a Muslim can commit, worse than murder (which the victim's family may choose to forgive).