There is always 2012 or 2016. I feel the Republicans will probably run a liberal to moderate in 2008. Guiliani is definitely on that list. If the Constitution is amended to allow non-born citizens to run, there is a good chance Arnold will run.
...although the chances of that happening are zilch.
I very much doubt Jeb Bush might run if his brother wins next month. That wouldn't just
look like a dynasty anymore.
If Kerry wins though, it's possible. Although 2012 is more credible than 2008. (Is there a term limit in Florida? Or could he keep on being Governor?)