Probably some third party candidate. Write in votes aren't even counted or reported. At least voting for a ballot-qualified third party will give them one more little vote that will actually appear on the radar. In the vast majority of states, you might as well write in Mickey Mouse or Cthulhu as much as Ron Paul.
Hell, none of the write in votes for Nader were even reported last time in all the states where the Democrats kept him off the ballot. And he was officially registered as a write in candidate, which Dr. Paul won't do.