I spent a lot of last year in the US , with an unlocked iphone , sim cards are available from many carriers , AT&T , T-mobile , and so on ,
The services available from each of these vary depending on the price plan , not on the iphone.
T-mobile will sell you a sim , and will cut it down to size for you , for around $50 with so many minutes of talktime and access to their 3G network.
You will have to talk to each supplier to find out exactly what you get access to as part of the package , prices and special offers vary almost weekly !
I was mostly in Phoenix Arizona , and Portland Oregon. Something to keep in mind , most restaurants , bars and public places like malls , have free Wifi .... so for all the haggling about the best deals for 3G access , I hardly ever used it as free wi-fi was so common.