I broke 2 shutter button, and I know another diver using a D90 and an Aquatica housing who had the same problem (Girelle).
My D90 is no longer under warranty, but my trick works.
The shutter button is made in a very poor plastic quality.
The problem comes from the trigger always pushing the exact same part of the button...
I explained my case to Aquatica and they provided me with a new trigger that I still have to implement to my housing.
send me a message with your email address on flickr and I'll doit.
Damien
We have the exact same rig !!!!