From what I've heard Jeb is also more conservative than W, so it seems unlikely that he would want to team up with McCain, though he might decide it's his civic duty to do so it the Democratic opponent is bad enough.
I really can't see McCain having a chance of getting nominated, so it seems like pointless conjecture anyway.
A liberal can win the Republican nomination in 2008 because the Republican base is more liberal than people think it is. Look what happened in IL this spring with that awful woman running for governor against that equally awful governor.