U.S. Senators typically make for abysmal presidential candidates, and Senate majority leaders are the worst of them all. If Frist were selected to head the GOP ticket in 2008, he'd probably make Bob Dole look like a champ.
If he doesn't seek re-election next year, and it appears he won't, he's got two years to distance himself from the Senate and he won't have any recent controversies to haunt him. And he might leave as a hero in the judges battle and possibly some important legislation.
It's just too early to dismiss him as a contender.