It depends a bit on what he says when he drops out. I get the sense Thompson is something of the southern wing of Romney, so him getting out w/o saying anything (especially while Paul, Huckasquirrel and Giuliani stay in) should help him... might be what's needed to prevent McCain actually. 
But it appears that if Thompson drops out, he might endorse McCain as the two are close personally.
Thompson endorsed McCain in 2000 and I believe he was actually doing some work for the McCain campaign this time around before he decided to launch (if one can use that word about the Thompson campaign) his own. As for his supporters, I don't know. I'd expect most of them to split between McCain and Romney.
