I think McCain will vote with the GOP. This isn't one of his media-driven issues and sometimes he really does the right thing. Susan Collins isn't as looney as Snowe and I think she'll vote rightly on this.
Frist can obviously count and he won't bring it to the floor if the votes aren't there.
Just my opinions.
Party caucuses enforce party discipline much more on procedural votes. This would be a procedural vote. I don't expect McCain to bolt.