palin and huckabee strike me as the most likely
Sarah Palin is overvalued. Her support is mainly a backlash against perceived mistreatment of her by the media. She is not a leader in the GOP as her only selling point is energy policy, and that is becoming less and less of an issue everyday (gas prices are averaging almost half of what they were when McCain selected her).
Huckabee is flirting with another run, but I think he likes his current gig and really resents the abrasivenss that is inevitable in a presidential campaign. It all depends on how his TV show is doing in two years, IMO. And even if he did get in, his appeal isn't that broad.