It really depends on the type of postbac program you're looking for. I notice that a lot of people posting don't specify whether they're looking for 1) a program for career changers and those with little or no science background or 2) a program for premeds who want to improve their low GPA or strengthen their credentials by taking higher-level science courses or getting a Master's. The GPA and test score requirements are different.
If you're looking for #1, average GPA and SAT scores for the highly regarded programs are 3.5-3.6 and 1350. I got into Bryn Mawr and Goucher's programs with a 3.7 and a 1460. However, for other postbac programs, the requirements are lower. For example, from what I've heard, you only need a 3.0 to get into Columbia's and Harvard Extension's programs.
I'm not so sure about #2, but others can help you out there.
***EDIT - Ooops, I just realized that your original question specified MCATs, so I assume you're looking for #2 types of programs. Sorry about that!***
