I've jokingly stated in the past a scenario in which Clinton approaches Republican leadership, stating she is ready to switch parties, seeing this as the only real means she has to gain the White House. While I still think this scenario is exceedingly unlikely, it would not surprise me at all.
I think it would be more likely for McCain to run as a dem, now that Kerry is history. Yeah, that's the ticket. McCain/Hillary. Ooops! I meant, Hillary/McCain.