FWIW, here's what Wikipedia says:
A theme of Obama's 2004 Democratic National Convention keynote address, and the title of his 2006 book, The Audacity of Hope, was inspired by his pastor, Rev. Jeremiah Wright. In Chapter 6 of the book, titled "Faith," Obama writes that he "was not raised in a religious household." He describes his mother, raised by non-religious parents, as detached from religion, yet "in many ways the most spiritually awakened person that I have ever known." He describes his Kenyan father as "raised a Muslim," but a "confirmed atheist" by the time his parents met, and his Indonesian stepfather as "a man who saw religion as not particularly useful." The chapter details how Obama, in his twenties, while working with local churches as a community organizer, came to understand "the power of the African American religious tradition to spur social change":
It was because of these newfound understandings—that religious commitment did not require me to suspend critical thinking, disengage from the battle for economic and social justice, or otherwise retreat from the world that I knew and loved—that I was finally able to walk down the aisle of Trinity United Church of Christ one day and be baptized.
It sounds like, if he has ever been even remotely Muslim, it's only (eta: because his father was, though not by the time Obama was born) in the same way that someone whose mother is Jewish is also Jewish (which may not apply to Islam at all; I'm utterly ass-ignorant about that).