A while back I blogged on this recent single center study which showed that a small, select group of students who bypassed the traditional premed basic science curriculum and the MCATs did as well in medical school as the rest of the students.
Of related interest, now there's this very large study looking at predictors of difficulty in medical school, particularly withdrawal or dismissal, and failure to pass USMLE on the first attempt. The most powerful predictor of difficulty was the MCAT score, and the lower the score the greater the likelihood of difficulty. This is difficult to interpret, because there was no breakdown of the MCAT categories, two of which are in the sciences and two of which are in the humanities.
The Skeptical Scalpel blog has an interesting take on both studies along with some general comments on medical education, here and here.