Will Smith may be a good actor, but not all good actors are able to inhabit very different characters. And I just don't see Smith pulling off a Barack.
Anyhow, I have a gut feeling that, seeing as how in our youtube age we've already seen Barack face most of his biggest personal moments/challenges, the first major Obama biopic will be a "Barack Begins"-type movie, focusing on his 20s, as chronicled in David Maraniss' new book Barack Obama: The Story. So, a young unknown/little-known actor of roughly that age'd be best.