I'm already part of an AT&T business plan, and would just as soon continue to use AT&T. Using T-Mobile with my contract-free G1 isn't an option, as they don't have coverage in my area. Also, I live in a small town where WiFi is available almost everywhere. It would be nice to have Internet access while commuting, etc. but I don't think I would use an unlimited data plan enough to justify spending upwards of $20/month for the convenience. For these reasons, I wanted to use my G1 on AT&T's network without purchasing an unlimited data plan. Here's what I did. Activating and Unlocking Upon inserting my AT&T sim card and booting the phone, I was presented with an unlock message. I dismissed the message, entered AT&T's APN settings and attempted to sign in to my Google account. As expected, it failed. With AT&T's APN settings set, all I needed was an unlock code. I purchased one from www.unlock-tmobileg1.com. They were fast - I got my code within hours of ordering it. I restarted the phone and got the unlock prompt. I entered the code, and it was accepted. With the correct APN settings still intact I tried again to sign in to my Google account. It worked! No T-Mobile SIM required. Disabling Data Now that the over the air activation was complete, I wanted disable cell data completely. As one unfortunate T-Mobile customer discovered, cell data can't simply be turned off. It seems that the G1 uses small amounts of data even in standby. While this isn't a problem with an unlimited data connection, it can get expensive if you're roaming or on a per kilobyte rate. My solution was to leave the username and password APN settings blank. That way, the G1 simply doesn't know how to use the data connection. These are the settings I used: Name: AT&T APN: wap.cingular Proxy: leave blank Port: leave blank Username: leave blank Password: leave blank Server: leave blank MMSC: http://mmsc.cingular.com MMS Proxy: wireless.cingular.com MMS Port: 80 MCC:310 MNC:410 APN Type: leave blank Voice, SMS, etc. still work fine. If I am not connected to WiFi and I try to access a webpage, or perform some other action that requires an internet connection, I get a network error.