I've been lucky enough to "play" with a Canon 50D this weekend, and I was really keen to try it in my Ikelite 40D housing.
And, unfortunately, after many hours of experimenting, I can report the THE TTL CIRCUITRY DOES NOT WORK
So....for all of you waiting for that information.....there it is.
It does not recognise the +/- compensation commands from the housing and just fired the flash at full no matter what the setting.
Same with manual settings. The flash fired at full power no matter what.
The camera certainly fits in the 40D housing, with no real problems.
A couple of the buttons are switched around - the 50D's INFO button is where the 40D's JUMP button is. Also the 50D's Liveview is accessed from the PRINT button (top left next to MENU).
I bit more intuitive to use than the 40D, it gives you a "quick control screen" which I found really useful, and I LOVE the extra pixels in the view monitor.
....aaahh well, it was fun to play.