The story so far - attempted to use CM9, which failed miserably. It simply would not take it - just kept rebooting. I've given up on CM9.
So I went back to the stock 2.3.
Then I tried another one: http://www.androidmerom.com/2012/02/05/release-androidme-sensation-v2-0-0/
Something went screwy with the PG58IMG.zip, as it came up with a CID mismatch error, and since then would keep rebooting between the htc and the htc "quietly brilliant" screens.
I tried to go back to the stock 2.3, but it still kept rebooting.
radionut seemed to have the solution, using AndroidRevolutions ROM: http://www.geekzone.co.nz/forums.asp?forumid=97&topicid=98208
But still, following his exact instructions, that stupid PG58IMG.zip still came up with a CID error, it rebooted on it's own, and has now been sitting on "beatsaudio" screen for 20 minutes. He said it would take a long time - that's too long.
Do I now have a brick?