I will be working in the US for five months. I have an iPhone 3GS from Rogers which I plan to unlock unofficially. What's the best way to get US voice and 3G data with this phone? Could I sign up for an AT&T smartphone plan and cancel it after 5 months? Alternately, does the AT&T pay-as-you-go SIM card still work? Any other carriers offering voice & 3G data for iphone?