In case anyone else is heading to the US for a few weeks with a smart phone...
I hunted around a few different options, thought AT&T would be a winner, since they had the iPhone themselves, but they're not. All their prepay options are geared at dumbphones, they have unlimited data but it's somehow disabled for smartphones and there's a $30 activation fee on top of it.
The winner (for me) was T-mobile, they have several monthly plans that are no contract, only $10 activation, and they carry microsims at all their kiosks:
1500 text/talk units + 30MB data
(if you run out of data you can buy a $1.50 daypass for 24hrs unlimited data)
unlimited text/talk units + unlimited data (first 100MB @ 4G)
unlimited text/talk units + unlimited data (first 5GB @ 4G)
I went with the $30 option as the remaining 10 days of the trip i could just buy a daypass anyday I needed and still cost less, and there's so much free wifi in the US it's not funny.
Beats the hell out of an international sim!