Here's a very sharp comment I found over at Gil's, written by a guy who calls himself Vladamir:
Nearly all Orthodox kohanim that I know never considered medical school, for the simple reason that they were kohanim, and did not want to violate "v'al kol nafshos mais lo yavo." So here's what I don't understand. Shabbos is more chamur than the prohibition of kohen contact with a mais. So why is the fact that Shabbos observance may limit one's options a reason to disregard Shabbos in favor of maintaining those options? If I had the skills to be a Major League baseball player, I certainly hope that I would be strong enough to maintain my Shmiras Shabbos, even though it would mean that I could never play shortstop for the Dodgers.
This is an awsome point. A full-fledged doctor has the right to violate shabbos when a life might be at risk (and we interpret this very liberaly; for example, he can answer the phone or drive to the hospital even when the odds are miniscule that the case is life-threatening) but a trainee doctor doesn't have life and death responsibility so the dispensation does not apply to him. A resident who knows that an attending or a senior resident is nearby, and available to take charge, is (likely) forbiden to violate shabbos. If he sits on his hands, no life is put at risk, so he has no right to act. The only out is a shomer shabbos residency and there simply aren't that many SS residencies available. Most orthodox Jewish doctors simply don't have the option of taking them.
Now, I'm not in the judging business so I don't much care that most Jewish doctors likely desecrate shabbos during their training. It's between them and God, after all. Still, I'd be very interested to hear about their experiences.