This problem is as bad for Apple as the contaminated milk problem for China, and it may very well be the first scandal of this kind in the computer industry.(Why are there so many great quotes and analogies about Apple?) The evidence on both sides is pretty convincing. One one side there's a researcher who's own lab colleuege is calling his science into question, (comments), and on the other side is a typically silent Apple with no comment. It's implied that they are aware of the problem, and are doing everything to fix it quickly, but also that they don't think that the problem is really a problem. (i.e. They know the computer smells funny, but aren't going to say it gives you blood cancer.) I, for one, would like to believe that.
A recall would probably be slightly irritating for Apple. (Making the sure bet assumption that they'll never prove Mac Pros cause cancer.) Apple had to recall 1.8million notebook batteries in 2006 due to a overheating battery. They were barely effected, but the manufacturer Sony took a several point hit. Will Apple be able to shift the blame this time as well?
EDIT: There is a semi-update on Apple's nonposition on this.
Apparently they're "looking into it" even though they don't believe it's true. Because that's what major corporations do when one person says something that everyone else basically thinks is nonsense... they launch a research campaign.