And now a grown-ups response to your question: I don't think Obama and Hitlary can coexist. If you look at some the sniping going on between the two of them now, there's already some bad blood and will probably get worse as we get into the primaries. They both lean pretty far left, but at least Obama is upfront about it, whereas Klinton is still convinced she can fool pretty much everybody.
As far as who would be the possible running mates on the GOP side? Best guesses: Condoleeza Rice, Gingrich (unless he does decide to run), Tommy Thompson, and I wouldn;t be too surprised if the name Jeb Bush comes up. Guliani will probably look for a moderate, possibly Dr Rice just for the drawing power.Romney will look for a more traditional conservative, and who knows what Fred Thompson is going to do.....