Earlier reports were that Collins, Warner, DeWine, and Graham would vote to go nuclear if the Democrats renege on the "agreement." So, the Dems are really the ones who will decide if the button is pushed.
I'm not so sure they'll play that card. Filibustering a Supreme Court nominee will be an order of magnitude more visible than the earlier appointments. They'll only do it if they focus-group it and find out that they won't get the public riled, and I'm guessing that they'll find out that the public won't be all that sympathetic.
The Dems are already telegraphing their strategy for the person to replace Rehnquist. There was an article posted yesterday by AP which basically took that tone that Scalia is just too mean to be effective to be Chief Justice. Hey, if it worked for Bolton, why not try it on Scalia? And of course, Thomas will be just too ethically compromised by the Anita Hill accusations. They might tolerate O'Connor, simply because she's so loopy any more that she votes with them half the time anyway.