AT&T offers data with their Gophone (prepay) SIM at 100Mb for $20 for 30days (it will rollover if you need to, but you have to add 1Mb for $5 at the end of 30 days to get it to roll over for another 30 days.
Prepaid cell phone, GoPhoneŽ - Wireless from AT&T
T-Mobile has a non-contract prepay month-to-month SIM offering a number of configurations including unlimited everything for from $50-$80 dollars for the month. It's called the "Even More Plus" plan, and yes, you can buy just the SIM, you don't have to buy a phone.
T-Mobile Even More Plus