Oh.
I think 95% or more will go to McCain. I have no historical stats or anything to back me up, just my hunch.
When I've been reweighting polls based on saner party ID numbers (reweighting to R/D/I of 36/38/26), I've also put in a check to see what would happen if the undecideds broke 2:1 for McCain -- in most cases, it puts McCain clearly in the lead after the party ID question is settled.