If I were a gambling kind of gal, I would put money on Lieberman as McCain's VP pick. Don't get me wrong; as a mostly libertarian at heart, this ticket sends a shiver up my spine, but that is until I look at the alternative. Lieberman is not my personal favorite choice for VP, but he makes sense for McCain. Picking Liebs reinforces both McCain's bi-partisan credentials and his desired maverick image. If fear of four years of Obama doesn't energize the conservative base, then there is not much McCain himself can do. Obama represents the Daily Kos/Moveon.org wing of the party so McCain can safely straddle the center and still be far to the right of Obama. After 8 years of playing Neocons vs. Hippies, much of the electorate might be sick of playground politics and willing to compromise on a centrist ticket.
Dick Morris has much the same analysis on a possible McCain / Lieberman ticket and additionally predicts an Obama / Joe Biden ticket. I don't think it's going to matter too much who his VP is. He definitely won't pick Clinton so regardless of who else he picks, his campaign is going to be all Obama all the time.