I agree with those who feel that the majority of Edwards voters will break for Obama if Edwards pulls out. I'd estimate something in the 60% range. I do think a lot of the women voters for Edwards would gravtiate more towards Clinton, however.
Thing is, Edwards isn't going to get out until at least February 6, meaning that at that point he will control perhaps 200 delegates or so. And if that happens, someone is going to have to offer him a VP slot in order to get him to offer up his delegates at the convention. This is in part why I think you see both Clinton and Obama not attacking him -- its in case they have to bring him into the fold down the road.
If things stay like they are now, it is entirely possible that neither will hve the nomination locked up heading to the convention. What will be interesting is whether Florida delegates -- who right now don't count because they moved our primary up and pissed off the party -- will somehow come back into play.