Ballot access would be a problem even if she were to try. A number of States don't allow people who ran in a primary and lost their party's nomination to run as a petition candidate. Not only that but there are already several states where it is too late now to get the signatures needed to run. Incidentally, even if Clinton somehow pulls a rabbit out of her hat, Obama could still be on the ballot in South Carolina. The United Citizens Party has nominated him for President.
I think I posted that here at one point. However, Obama said at the time that he'd reject the UCP's ballot line if he didn't win the Democratic primary.
There are all sorts of states Clinton couldn't be on the ballot in, including very large states such as Texas and Pennsylvania.
