I don't think there's any way with anything like even 50 percent certainty to guess the GOP nominee at this point. However, there is one other factor I haven't seen discussed that may favor John McCain: endorsements from his opponents.
It seems to me that if Mike Huckabee, Fred Thompson, or Rudy Giuliani were to drop out of the race, all of them would endorse McCain. Mitt Romney...well, would he endorse any of the four? None of them can seem to stand him, and perhaps the feeling is mutual.
It's unclear who would drop out first even if we knew the results of the several upcoming states. It seems Thompson will drop after South Carolina if he doesn't do very well. Giuliani admitted the huge importance of Florida to his campaign, and with him running low on money, it's hard to see how he'd stick around if he doesn't win or come in a close second there. Romney may bow out after another disappointing Michigan showing, but lots of people are speculating he'd stick around to try to rise above a poor field. Huckabee may have the longest shelf life, but if he suddenly drops and does poorly in both Michigan and South Carolina, he may be forced out too.
So in general, it seems like the more people drop out, the more McCain benefits. It's hard to believe for a 2000 McCainiac like me.