* If didn't jailbreak with Blackra1n like in this thread and used PwnageTool then you want to use this method to unlock your iPhone.
Once you are done with the jailbreak, you can now unlock your iPhone running jailbroken firmware 3.1.2 by using blacksn0w RC1 from Cydia. To do this:
Start “Cydia” on your iPhone.
Touch on “Manage” tab on the bottom.
Now touch on “Sources” as seen in the screenshot below:


Cydia will now automatically update your sources by following a series of automated steps.
After installation is completed. Search for “blacksn0w” in Cydia and install this app . This app will automatically unlock your iPhone so that you can use it with any carrier.


Now simply restart your iPhone. Voila! You are now running a fully unlocked and jailbroken iPhone on firmware 3.1.2 on your iPhone 3G or 3GS.
*** NOTE If your not running on firmware 3.1.2 the Check Options below !
** These directions are not necessary if you restore and/or Jailbreak with Blackra1n RC3
*
Thanks P !
You can also get a data plan from t-mobile and enter custom setting to get it to work which is a little more complicated !

Louisville, KY
Single & Looking
If you are using T-Mobile make sure you go to Settings and turn off 3G. Settings- General- Network- Tap to turn off 3G.