If you are the owner of a new iPhone 3GS then you would be looking for different ways to Unlock it. Thanks to the iPhone Dev Team which has now successfully unlocked the 3GS version using Ultrasn0w Application. Here is a video which gives you a complete demonstration of this process along with the text version.
1. First of all take a complete backup of all your data using the iTunes.
2. You need to have Cyndia installated on your iPhone.
3. Then disable 3G connection on your iPhone during the unlocking procedure since its not supported.
4. Install Ultrasn0w from Cyndia by entering the Cydia/APT URL using the Copy paste function.
5. Once this is done you need to return to the main screen and let the installation procedure get completed.
6. Once done you need to replace your current SIM card and insert a new SIM, ie removing the AT&T SIM and using T-Mobile one.
Check out the Video for Complete Demo!