Commenters on this blog have asked about two possibilities: Rudy Giuliani running for president as a Democrat and Condoleeza Rice running in '08. I think both are unlikely. Rudy spent much of 2004 campaigning hard for Bush and denouncing Kerry as a weakling. I think it would be very hard for Rudy to switch parties after that, even if he is closer to the Democrats on many issues.
Condi has four major problems as a presidential candidate:
1. She has never run for public office and apparently isn't all that conservative on issues apart from foreign policy. She has shown little interest in domestic politics.
2. She is a 50-year-old unmarried woman. While I won't engage in rumor-mongering about her sexuality (and such rumors seem unfounded anyway), it's hard to see a "family values" party nominating a bachelorette for president. ("Spinster" seems ill-suited for someone as stylish as Condi).
3. She has become so close to President Bush, that she has little image of her own. Although given Cheney's disinclination to seek the top job, Condi could fill the vice-presidential "designated successor" role.
4. Cabinet members tend not to make strong presidential candidates, due to their distance from electoral politics.