Why are you interested in paying $200/month (or more) to keep service with a US carrier while you're overseas for more than 6 months a year?

Apple just announced that unlocked (legitimate) iPhones will now be sold:

http://store.apple.com/us/browse/home/shop_iphone/fa ... »

Get an AT&T SIM to use here and get an overseas SIM from Grenada to swap out when you get there.

We can't provide you a rational solution to whole-heartedly irrational scenario (keeping US coverage while living overseas). If you plan to make more than 60 minutes of calls a month, a pre-paid international SIM will cost you 1/10 what carrying AT&T overseas will.