I thought about that a little bit. If Ike sold a D90 backplate, perhaps it costs $500. After all, it probably takes almost as much work/money/time to make that part as the front part because of the electronics and the number of machining and assembly operations.
You could sell your old D80 housing for $700 (a good deal for somebody out there) and buy a new housing for $1200. In the end, you spend the same amount and somebody gets a good deal for a D80.
I'm hoping to use my D80 until there is a D400. I thought I wanted the D90 but decided that it wasn't enough of an improvement. Liveview is marginally useful underwater and I think that the movie mode would be useless without autofocus. If you are using any kind of zoom lens, you would have to chose between having the zoom function or the manual focus function. I can't see dealing with manual focus underwater.
David