Charlie Crist, Bobby Jindal, and Tim Pawlenty will all be top tier candidates or have te potential to be. My hunch is that Crist proves to be the best of the three.
Jeb will not run, nor would he be all that strong a candidate if he did.
Gieliani will also have a strong case under the right conditions. Let's say Giuliani continues to be the only GOP candidate who polls well in GE matchups, and we nominate Romney or Thompson anyway. They crash and burn. Won't it become immediately obvious to everyone that the GOP should have nominated Rudy? Won't we regret not having done so as soon as Hillary takes office? Isn't he the immediate frontrunner in 2012 if for no other reason that people will regret not having nominated the only guy who could have won? And since every big time Republican will have shot their wad in 2008 and we have a bunch of Governors with no national security credentials running in 2012, Doesn't the race suddenly become Rudy versus the seven dwarves?
Even if Rudy were to become the nominee and lose, if it were a close race (e.g. 2000), he would probably become the 2012 frontrunner, as he could conceivably win in a more GOP-friendly year.
If Rudy runs and loses in the general, Republicans will not consider hium the front runner in 2012. We will have compromised on abortion, an issue that for many in our party is a black or white moral issue, and gotten nothing for it. If Rudy were pro-life and ten years younger, he could run well in 2008 and lose and blame it on the anti-Bush tide and then come back in 2012. But since he'll be 68 in 2012 (People forget how old these baby boomers are getting) and he's pro choice, he gets one shot at a general and only one shot.