T-Mobile is basically the only US carrier that seems friendly towards folks with unlocked phones just looking for a SIM card. There's also been some rumblings that they might start offering the first prepaid data service in the US; how likely that is, I don't know. I do know that they used to let iPhone users use their prepaid data plan for Sidekicks, but that is no longer possible. They only blocked port 80, though, so you can still check your mail via IMAP, or SSH to a remote sever somewhere! I've even used Opera before, by passing my traffic through an Opera proxy server.
Also, you can get $100 of prepaid credit for $70 by utilizing Bing.com cash back (YMMV) -- and that credit is good for a year. Not bad if you don't do a lot of talking or need port 80.