There have been many theoretical tickets, but none quite as interesting as this one. Not only would a Kerry-McCain ticket pull the usually Democratic vote, but it may as well pull more Independent and Republican votes from the Bush-Cheney ticket.
In addition to that, it may also force people to view Kerry as uniter, and that is certainly what the voters would like to see in their leader.
Of course, I highly doubt the likelihood of such a ticket. However, this would definitely be my favorite to go up against Bush-Cheney. Current polling on MSNBC shows that it would be a very strong ticket to take out BC.
http://www.msnbc.msn.com/id/4542473/