Here's hoping that Aquatica (or possibly other manufacturer's, but since I use Aquatica.... ) will have an option to purchase a new backplate, instead of a whole new housing. I imagine that just the back plate will be $1000, but that'd be better than re-buying the entire housing to go from 5D2 to 5D3.
Having said that, I'm not sure if the 5D3 will offer enough of an upgrade to be worthwhile. If they had updated the 1/200 sync speed, then it'd be a no brainer...