Megan McArdle and I debated this question, among others, in our recent Bloggingheads appearance. You can watch that particular back-and-forth below.
(Short version: Megan thinks our brave new health reformed world will cause lots of people to pass on being doctors in favor of becoming lawyers and middle managers. I think doctors will enjoy enough social prestige that we won't have trouble filling hospitals even if we pay them slightly less. We ended up in a bit of a stalemate over the empirical question of how much declining doctor pay has, in the past, affected people's desire to go to med-school. I'm going to try to dig up some evidence to shed light on the question. Anyone with a study to point me to should feel free to point away in comments.)