It didn’t take long! Folks at XDA-Developers have already managed to release a HardSPL package which unlocks the T-Mobile version of the HTC HD2. Given it is a very popular device it is not surprising that developers spent time unlocking it. Now you can use custom ROMs on it and truly leverage this amazing device.
Note that I won't be responsible if you end up bricking your phone, unlocking or flashing your phone can be risky. If you follow the instructions then you will be fine!
You can find and download the HSPL package for the T-Mobile HTC HD2 @ http://forum.xda-developers.com/showthread.php?p=5983094#post5983094.
Please also note that this HardSPL package is specific to the HTC HD2 with 1Gb of RAM (1024 Mb or LEO1024), you can find instructions on how to unlock other versions of the HTC HD2 @ http://www.techhackz.com/2010/01/hardspl-package-for-htc-hd2-was-just.html.
Just in case if you haven’t seen the T-Mobile HTC HD2 in action yet, here is a video from Wirefly: