I visited the US last year for two weeks (and going back this summer for three weeks), and I say, don't bother. You won't be using the phone as much as you think. If you are disciplined.
I try not to use my phone browsing unless I was on a wireless, which meant at the hotel or other hotspots. Fine, so sms cost $0.75, but I didn't send that many messages anyway (send email while at the hotel where you are going to meet your friends, then one sms if they were late). Probably didn't send more than 10 during my stay last year. Only thing was the map, maybe. But I got a Nokia, which meant I just pre-downloaded the map when I was on the wireless, and then tried to only use gps (no gsm). Not sure if iPhone has something similar.