Hey,
I took the medical school biochemistry and cell biology course here at WashU [Molecular Foundations of Medicine]. It was a tough course to take, being essentially two semesters of undergraduate cell biology and biochemistry scrunched into one with some clinical correlates.
I took it because it was a graduate-level class that fulfilled the requirements for my major.
I also took a class in molecular oncology, but most students were either PhD students or Heme/Onco fellows.
I've heard many undergrad students at WashU taking gross anatomy or MFM without trouble, but Ceez is right -- if you take a medical school class, do so because of interest and because you're sure of handling the pace and the subject material [usually familiarity or having taken a similar, easier class will do it]
Yours,
Jason