If you are looking to unlock the HTC HD2 you recently bought from T-Mobile, XDA Developers has released a HSPL2 which will allow you to flash the HTC HD2 and unlock it.
After unlocking users is free to use it as they wants. The only problem is it voids the warranty and any firmware upgrade from the official site will again lock the phone. So use it at your own risk.
Instructions to download the HSPL2 and flash the HTC HD2 for T-Mobile is available on the XDA Developers site.