I actually have a theory that McCain would love to pull out the nuke card. That is, if he is interested in running for President. It would, in his mind, help him get back in the good graces of the party base.
I hear you on that. McCain, Graham, and Co. would like nothing better than to be able to have an excuse to break the deal. After, McCain is an opportunist if nothing else. And he does want to be President some day. No worries from me, though. If the seven Dems break their part of the deal, the nuclear option can and will be used to get Bush's guy on the Supreme Court. We already have the votes to do it if the Dems break the deal and they know it.
Then again, he'll probably pull out some lame excuse like "Even though the Democrats are filibustering, changing the rules isn't the answer. The President should withdraw the nomination and consult with the Senate on appropriate candidates."
To be blunt, the McCain Mutineers will look for any excuse to go along with their Democrat friends like Chucky Schumer - as Lindsay Graham is doing already.