The chances are extremely high that Hillary Clinton will president after Obama or Romney. It is inevitable.
I think its likely that she will run (there is no way that she can pass it up with these numbers she has) and if she does, she'll probably be the nominee. But its far from inevitable that she becomes President. Democratic fatigue, a good GOP candidate and the return of Hillary as a polarizing figure (because I don't think the GOP will like her when she's a candidate again) will make it harder for her to win.
