HTCDev keep on adding bootloader unlocking for different variants of Sensation 4G phones. Recently, HTC Sensation users holding European version of the Sensation were the first one to have the honors but now within few days T-Mobile version of HTC Sensation has also been added in bootloader unlocking list at HTCDev.
Once again HTC makes clearly says that this is an experts-only affair:
We strongly suggest that you do not unlock the bootloader unless you are confident that you understand the risks involved. This is a technical procedure and the side effects could possibly necessitate repairs to your device not covered under warranty. If you are still interested in unlocking the bootloader, and you understand the consequences both to your device and to your warranty, then you may refer to the following pages where we have provided the unlocking instructions.
HTC is committed to listening to users and delivering customer satisfaction. We have heard your voice and starting now, we will allow our bootloader to be unlocked for 2011 models going forward.