Ali Eteraz refutes at the Huffington Post:
people that appear to be Muslims, but don't follow Islam and choose another religion, are permitted under Islamic law to leave Islam without penalty. A major case in Malaysia recently handed down -- a woman who was Muslim for some time in order to marry an Iranian was permitted to go back to Buddhism -- is an example. Obama, unlike the Malaysian woman, didn't even make a profession of faith to Islam, so it makes even less sense for him to be considered an apostate.
No call to prayer in the ear, not raised as a Muslim, born to an atheist father, and then abandoned to a Christian mother both by father and his family, equals not Muslim. Obama is right to say he had no religion until he became a Christian.