Paul has to poll better. His media exposure spiked when he finally rose in the polls in Iowa. Obviously he doesn't receive as much air time as Newt and Romney do, but he doesn't poll as well as them either. If Paul appears to be a threat to win, he will be given plenty of air time.
I believe one of the reporters on CNN said that he was not attempting to vote again, he just wanted to be allowed entrance to observe the process. I think he actually ended up being let in as an observer.
I'm just being realistic. McCain has too many delegates and too much love from the media to be derailed at this point. It would take an act of God to stop him... and I'm agnostic so I doubt we're going to see said act.