Right now Thompson is supporting McCain, but who knows...
If he thows his hat into the ring, it will be to get name recognitin. Time wil tell
AFAIK, Thompson hasn't officially endorsed McCain this time around, although he did endorse him in 2000 (after Lamar Alexander dropped out) and became his national co-chairman.
The two are good friends, and Thompson probably doesn't want to rain on Mcain's parade. However, with McCain slipping badly in the polls, that reason for staying out could fall by the wayside.