AlphaRevX, the same team that unlocked and provided full S-OFF for the Incredible S, Desire (GSM and CDMA), Wildfire, Aria, Incredible 2, and Desire S, are back again in a big way: with a full unlock, S-OFF and permanent root for the Sensation. Have a look:

AlphaRevX have already stated that they have full root and S-OFF for the EVO 3D as well, although we're not sure if it uses the same method, as the download has yet to be released.

4Erou

AlphaRevX is a software HBOOT mod, which is completely reversible and provides a software S-OFF. It lets you root, flash custom recoveries, nandroid back up/restore, and flash custom ROMs - basically, anything your heart and soul desire. I'm not positive whether this gives you write access to /system from within the stock ROM, but in either scenario, a custom ROM would definitely allow that.

To keep up with the latest info on what's going on with this, hit up this thread on XDA.

[XDA 1, 2]