Look at it this way: if you're in a program that enables you to take classes with (or graded against) medical students with their average set to your B and you're earning a 3.0 GPA, you're beating half of the admitted students. For medical schools that are familiar with well-established programs (Georgetown SMP, Drexel IMS/MMS, etc), my feeling is that something in the overall B range will get you get you some consideration but probably not during the SMP year. Of course, having a 3.5 or better will earn you more attention and (hopefully) more acceptances.
For programs in which you don't compete for grades, the relative value of a higher GPA decreases, as there might be artificial grade inflation or competition on a curve against students who might not be operating at the same caliber as medical students.
Now for my unsolicited two cents. For someone considering enrolling in an SMP with a poor undergraduate sGPA: be careful. I did just that and while I'd be doing fine and dandy as a medical student with these grades, I am falling a percentage point or two behind the medical school mean (usually in the mid-80s). This might be closing more doors than opening at this point, which is a shame considering how much I study and how much this program costs.
Best of luck!