How can I unlock AT&T iPhone 3G to use with T-Mobile?

The Video:

I bought my 3G iPhone from here

You can find the unlock SIM cards here

Unlike the first generation iPhone (2G), currently there is no software unlock solution. However, there is a hardware solution. You can buy a very thin unlock SIM card that can be placed on top of your T-Mobile SIM card and insert both into the iPhone 3G. This unlock SIM card will make the iPhone 3G to believe the T-Mobile SIM card is either a Demo card or an AT&T card and thus allow you to use the iPhone 3G with other carriers.
Pro:
· No need to modify the firmware, or jailbreak the phone
· Will not void your warranty
· You can sync with iTunes, 99% of the applications works, including YouTube, Google Map, etc.
Con:
· It does make the SIM thicker, which could be a bit harder to eject the SIM tray
· Newer version of iPhone firmware might detect old version of SIM unlock card. So you either have to buy a new SIM unlock card to never upgrade your firmware
· App Store or any application that uses HTTPS will not be access T-Mobile data network unless if you jailbreak the phone and modify some configuration files. Of course, these applications will always work over WiFi networks. So if you need to install any applications from App Store, just do it via WiFi.
The unlock SIM card
You can find these unlock SIM card from eBay


From left to right: T-Mobile SIM, the unlock SIM, and the SIM tray.






From top to bottom: The unlock SIM, the T-Mobile SIM










The iPhone 3G with T-Mobile SIM and unlock SIM.
The Settings:
To make phone call, you have to
· Set Settings->General->Network->Enable 3G to OFF.
To access to the T-Mobile internet data network with $19.99 monthly plan, you have to:
· Set Settings->General->Network->Enable 3G to OFF.
· Set Settings->General->Network->Data Roaming to ON.
To access the T-Mobile WAP internet data network, (T-MobileWeb $5.99 monthly plan), you have to:
· Set Settings->General->Network->Enable 3G to OFF.
· Set Settings->General->Network->Data Roaming to ON.
· Set Settings->General->Network->Cellular Data Network->APN to epc.tmobile.com
We would highly recommend the $5.99 monthly plan, which gives you unlimited internet access at the Edge network speed.
The unlocked iPhone 3G with $5.99 T-Mobile data plan:



I bought my 3G iPhone from
here