Well, the way I see it, MMA is becoming so big in America, that really no matter where you live, you can get the proper training, and be within a few hours driving distance of an event-holding venue. It might increase your chances a little bit if you were stationed in Japan, Brazil or possibly Hawaii (where MMA is apparently becoming huge), but I wouldnt say enough so to warrant joining the armed forces on its own. But if you're gonna do it anyway, and can get stationed in one of those places, it certainly wouldnt hurt.

I have a friend who teaches and competes in BJJ, and teaches "MMA" classes, and he has quite a few students who travel a few hours a couple times a week from Camp Lejeune (I live in Central, NC...Lejeune is way out east somewhere...between 4-5 hours, depending on how you drive).