Since I was in the US 3 weeks for this trip I purchased a T-mobile Sim. It was a prepaid all you can eat (Unlimited Talk + Text + Web) plan and I paid almost 100 USD including fees and taxes for it for unlimited text, unlimited US calls (in and out) and unlimited data. Considering the steep roaming charges and considering the use of things like http://rebtel.com for cheap or free overseas calling that was a reasonable price to pay....
If you are only after data and can live with 2.5 G internet AT&T offers data as a add on to pay as you go. I think it is around USD 20 for 100 mb