Nightmare

Is John McCain planning on bailing from the Republican Party to run as Hillary's VP?

This would throw everything out of whack.  What would REALLY throw everything out of whack would be Hillary and McCain both bailing their respective parties and running as the independant ticket.

Imagine: the democRats, under Howard Dean, move further left, nominating a ticket like Boxer/Kucinich.  The Republicans, getting too cocky, nominate a controversial conservative Christian ticket, like Ashcroft/Lott.  Hillary and McCain, both feeling like their parties are heading the wrong direction, team up and move for the center.

They would win. 

Not by much, and certainly not with anywhere close to a majority (probably not even as high a percentage as Bill Clinton won in 1992), but all it would take is 34% of the popular vote with even distribution.

Not that I expect any of this to actually happen.  It's just a fun little exercise in "what if?"