I served in the peace corps in Senegal West Africa from 99-02 as a health volunteer. I had joined with the intention to go on for a MPH degree, but the direct patient contact experience I garnered led me to medicine instead. As I had been a philosophy major in University, I have had to take the last couple of years working and completing ALL of the prerequisite courses. I'm currently stuck right into the 2005 application cycle, having just taken the MCAT's in April (36s).
That's my background. So you should believe me when I tell you that going into medicine after two and a quarter years in the bush is entirely possible and common. Of just my group in Senegal, 3 out of forty of us are either in med school now, or trying to get in. You won't be alone.
As far as logistics goes, there are many routes to the same goal. First off, decide if PC is right for you. It's quite a commitment, and choosing to do 27 months w/out pay or much western contact is very difficult. If your only reasons for doing this are to put down the supposedly "ultimate extracurricular activity" onto your med school apps, you probably won't make it. But if you have the motivation for it, you will recieve broad based exposure to **** you can't see in the US.
From the experience of my classmates, the MCAT's should be done after/during your time. You are allowed two days off per month in the Peace Corps, and can accumulate a few months worth of time to take ten days back in the US (very common for the homesick). I have a friend who did just that, scheduled her MCAT during this vacation, and studied in her village for a few months prior. She just finished her first year at tufts. You could also just wait till you come back to take the test, which of course gives you precious time to readjust to the culture of "this american life". You'll probably want a few months just to decompress.
I hope this post (being a bit too long) is informative in the very least. Feel free to email me with any specific questions you may have.