As with iPhone 2.x settings, the T-Mobile internet settings are in General -> Network -> Cellular Data Network. For Cellular Data:
Username or Password are blank.
If wap.voicestream.com does not work, you can try internet2.voicestream.com
The following settings seem to work for MMS messages with T-Mobile USA (I’ve been testing them for more than a week) and they are listed below the Cellular Data settings under MMS:
You may have to restart the phone for the MMS settings to take effect. You will see a little camera icon below the text field in the “Messages” application.
Have any more tips?
Next up, tethering your computer via your iPhone on T-Mobile. 😉
It looks like you can do it with the instructions here for not just T-Mobile USA, but other carriers: