Talking horse race only here, not my personal politics:
Condi has no real chance for the Republican nomination right now. About the only scenario I could see is that Dick Cheney resigns for health reasons, and she gets appointed and confirmed by the Senate and thus becomes the heir apparent. She'd never make it through a Republican primary process at her current level. I'd love to have seen her take over for Cheney in 2004, but figured when she didn't that Jeb was running in 2008. Looks like that's not going to happen right now.
The most likely scenario is for her to run for Governor of California next time around and see what happens from there. She is also doing nothing to indicate her interest in running in 2008--no visits to Iowa and New Hampshire, for example.
I have assumed that the Republicans would be the first to nominate a woman or a black to the top spot, for the simple reason that it's less risky for them; they already get a majority of the white male vote and that's not likely to decline just because a woman or a black is representing the party.
But I have also long believed that Hillary will be the Democrat's nominee in 2008, and I see zero reason to conclude that's not going to happen outside of Mark Warner. Warner is trying to position himself to the right of Hillary, which means he's targeting an apparently small portion of the Democratic base as of now. Gore would have an excellent chance at the nomination if he could narrow the field to that threesome. Dark horse (which usually means favorite among the Democrats): Russ Feingold.
Hillary's gotta run; the next election in 2012 would have her defending her Senate seat at the same time. Condi doesn't have to run; she's got time to prove herself in an executive position. McCain has to run; he's too old to wait around and he's doing all the things he needs to mend his tattered right wing. Repubs tend to go with the guy whose turn it is; Ford in 1976, Reagan in 1980, Bush, Sr. in 1988, Dole in 1996. They never go with some guy out of nowhere. Giuliani is the other possibility, but I get the sense he's hoping for the VP slot or the 2012 nomination if the Republicans lose.