For self-reference, and to save others time:
Got a friend's iPhone 3G going on T-Mobile. iPhone 3G, iOS4
Jailbroke with redsn0w
Unlocked with ultrasn0w
Changed T-Mobile data feature to the $10/month unlimited web2go plan - told the rep that I had a Samsung T509 crap-phone.
At this point, I had an issue with not being able to set the APN. It just wasn't an option in the General->Network menu on the phone. Attempting to access websites without changing the APN resulted in an error message from T-mobile goading me into a $25+ smartphone plan. I used the "email" option with www.unlockit.co.nz to set the internet2.voicestream.com server as my APN (I believe it was the first T-mobile option on unlockit)
After setting the APN, everything worked.
Regret on the jailbreak: Enabling background wallpaper, multitasking. iPhone 3G is painfully slow/laggy as-is; it needs to run as lean as possible.