Mrsimcard.com has a prepaid SIM for the iPhone with unlimited ATT data, but you are paying $70 a month for this service. iPhonetrip.com has a global simcard that they will rent you with a US number, some US minutes, and and unlimited global data, but you are paying $79 a week for it.

The Nexus is only helpful on prepaid in Canada. You can put it on the new Wind network. Public Wireless is opening its doors today and may have an offering on the 1700mhz AWS band. Google is promising a 1900/850 Nexus, but before running out and buying it read the reviews about Google has dealt with the poor 3g reception issues on the 1700mhz Nexus. The answer is that they just said it wasn't their problem.