Something I hate more than anything else in the world is a software lock on hardware I own. If I bought something I should have the right to do anything with it that is within it's physical capabilities. Earlier this year I wrote how to get Android running on your HTC Tilt, if you followed that guide (or one like it) you will need to flash your Tilt back to Windows Mobile in order to unlock it.
All you need in order to unlock the device is a foreign (non-AT&T) sim chip and the willingness to partake in a little bit of social engineering. First things first, power down your Tilt and pop the battery out and write down the 15 digit IEME number of your device. Next, from another phone that is active you are going to want to call the
AT&T International Contact Number - 1-916-843-4685
They are open 24/7/365, so call whenever you please. If you have a current account with AT&T (or know someone that does that will let you use their information) you can get to an operator much faster. Once you finally get a real person your call should go something like this:
Me: Hello there, I am going to be doing some traveling outside the U.S. and I need the unlock code for my AT&T Tilt phone so I can use it in other countries.
Operator: Hold on just a moment let me see if I have access to that unlock code.
Operator: Yes, I can provide you with that unlock code. I will need the IEME number listed on the back of your device under the battery.
Me: Provide the IEME number
Operator: Worked like a charm, here is your unlock code. Is there anything else I can help you with today?
Me: No that is all, thank you.
After you have the unlock code, simply power up the device with the foreign sim chip and it will prompt you for an unlock code automatically. Enter the code you where given and poof! You are all set to go, your AT&T tilt will now work on any GSM network.