My Sensation is running 2.3.4, why hasn't this updated?
Android 3.x is reserved for Tablets, and so would never be running on an actual phone. As for 4.x, it's basically a waiting game. If there have been assurances from HTC that the update will eventually be available for your phone, you'll just have to wait until your phone tells you it's available.  The only way to speed things up would be to monitor the forums and see when others have gotten the same update.  Then you'll have a chance to download it yourself and tell your recovery to install it.  Even then its still a waiting game.