SIM card w/data for USA iphone 3G

I am going to the USA for 3 weeks and want to have a SIM card that I can put into my old (unblocked) iphone 3G to have local data and phone service.

Has anyone done this successfully? Know what is the best deal for around 3 weeks of service, minimal use (emails mainly) but reliable access and decent speed?

I have heard that their are problems with both AT&T and T-mobile data plans and GSM phones. Anyone know more details from experience?