It seems HTC has made good on its commitment to begin unlocking the bootloaders of its recently released handsets, starting with the European version of the HTC Sensation. You can find the instructions at HTC's developer portal, here. (You may need to log in). Also, here are the scary things you'll have to disclaim before beginning:
So, as long as you realize that your warranty will/might be voided all or in part, get cracking. HTC's instructions are several pages long, and pretty in-depth. You'll have to get the Android Device Bridge (ADB) up and running, as well as HTC Sync (for Windows users) and the latest version of Java. Essentially, HTC has you retrieve your unique device "token" and submit it to them, at which point you follow additional instructions to complete the unlocking process.
Again, this is only for European Sensation users at the moment - your T-Mobile Sensation will not unlock by these instructions at present. Let us know how it goes!