The only chance McCain has to even run for President is to cast a YES vote to the eventual "nuclear" option. If the Dems filibuster and McCain falls in for this option, he can claim an attempt at moderation while pretending to stick to an up or down principle. The only, and very slim, chance would be to lead on a conservative victory.
Which is fully what I expect will happen, and what I believe McCain is banking on. By aligning himself with the gang of 14, McCain has accomplished several things. He's increased his stature among moderates and independents as a "reasonable" politican. He's undercut Bill Frist, one of his top contenders for the 2008 nomination. And when the Democrats break the deal and filibuster Bush's first Supreme Court nominee, John McCain can ride in on his white horse and save the day for conservatives.