Now, Will Smith is the obvious answer. Obama even said so himself because, and I'm not making this up, "he already has the ears". A few more pieces will fall into place because Smith's wife and two daughters (that is a girl in Karate Kid, right?) round out the First Family almost perfectly.
However, I'm a fan of making movies on a budget. Will Smith brings along quite the hefty price tag. Besides, this movie is going to be all about Osama, so Obama can't over shadow him that much.
So a cheaper, and arguably better, alternate to Will Smith playing Obama is...
The role of Biden would have to go to Bob Barker, of course.
With Kel as Obama, you don't even need to hire writers, this dialog writes itself.
Biden: Ok Baracko, grab some socialized healthcare and a copy of your birth certificate, we're going to Afghanistan!
Obama: But Biden, wait! Biden! We don't have socialized healthcare. Biden! Come'on man, Biden! A...a monkey ate my birth certificate. Biden! Hey Biden! I don't even know where Afghanistan is. Biden! Biden! BIDEN! Awwwwwww man, here it goes!