Re: Cell phone usage for USA and "maybe" Caribbean
I can't help you on the Carib side but on the US, Verizon DOES have a few 'global' phones that will do GSM and CDMA and work outside the US. If you really want to stay with them.
But if you are heading out, I'd suggest looking into Straight Path (only available at WalMart) or one of the other new "unlimited no contract" carriers. They have only recently started carrying smartphones and have a limited selection, but you're looking at $45-50 per month for unlimited everything. SP uses the Verizon network as their backbone, so the coverage is about the same. MetroPCS is even cheaper, not real national coverage yet. There are a lot of options that would pay off over just a 5-month period, and give you the chance to look at the US market afresh when you come back.
Meanwhile, that GVoice number means you give out one number, and no one needs to know if you've changed carriers or whatnot behind the scenes.