Aquatcech have unveiled their new housing for the Canon 5D Mark 3 today, the DCV-V3. Essentially the design remains unchanged from previous versions but of course updated to account for the new button layout of the 5DMK3. You can also purchase an upgrade kit to convert both your previous DCV-v2 5D MK2 housing, or your 7D housing.
Price looks to be $1895
I think the only put off for these is the price, at least for me.
If you have a 60D or 7d then they make a simple housing for that which is only $900.
Just took a look to see if they have one for the 50D, though no luck