Romney is a smart guy and a realist. It just wasn't happening for him. In the end, Huckabee seemed to be a bigger problem for him than McCain.
I doubt McCain will offer him the VP. He's probably already promised it to 4 or 5 other guys. Romney never developed any traction, so it's difficult to see what he would add to the ticket other than intelligence, good judgment and competency, which McCain obviously doesn't value anyway.
This gives McCain a golden opportunity today at CPAC. By being gracious toward Romney, he might mend a few fences. He needs to make a couple of Read My Lips type pledges on taxes and immigration.
Say, it looks like Ron Paul is now one of three remaining candidates. Since Huckabee stayed in the race only as a McCain stalking horse, I wouldn't be surprised to see him drop out soon as well.