T-Mobile has a unlimited WAP data plan called T-Zones that’s $5.99 a month. Unlike the normal $20/mo full-internet package, the T-Zones service blocks ports 80 and 443 (the default http and https ports).
There is, however, a T-Mobile WAP to HTTP proxy server that will (assuming your device is configured correctly) allow you to browse the web on the cheap plan, and this is just what Windows Mobile users have been doing for a while to get affordable, unlimited, mobile web service. Now that the iPhone is easily unlocked and usable with T-Mobile’s service, you can do the same. You just need to configure your iPhone to use the proxy server. It’s a little slower, but at 6 bucks, it’s probably the cheapest data plan available.