I think he's probably agnostic if you want to get technical about it. I certainly don't think he is a Christian or Muslim. He is an astute politician though, which means he has enough sense to realise that publicly stating what he doesn't believe would be electoral suicide in the US.
Well the reason why I said he's probably a very tolerant Christian rather than a non-believer is because of his words about having faith and it's importance. I doubt any Agnostic would talk about faith--they may believe in some sort of 'god' but the rejection of faith is something Atheists and Agnostics have in common. Judging from the video, I doubt Obama is an Agnostic for reasons I already stated.