I wouldn't totally rule it out, though I think it's unlikely.
But more importantly, it would backfire. Kerry/McCain wouldn't cut that deeply into Bush's base. The Mc is well-exposed as a RINO, and such party jumping would be absolutely unforgiveable for most Bush supporters. Meanwhile, his strong pro-life positions and general machismo would drive the feminist vote to Nadar, who might get 10+%. Mc is considered an unreliable, proabably dangerous nut by almost everyone except a few middle-of-the-road Imus-watching IQ-challanged dolts.
Well as I said I made that prediction 2 months ago and got all of the arguments I am seeing on this thread. It is odd that McCain is the ONLY one being prominently mentioned in the media recently. I see a lot of short term poll boosts by doing this and it would be THE main story for weeks leading up to the Democrats convention.