I think this is a very interesting discussion. It is important to remember that what will happen to the camera-market and what is best for underwater photography are two different things. We're a pretty small part of the overall scheme of things and of course, the camera companies won't often consider our wants/needs.
I think DX has plenty more legs for now. I am sure Nikon has sold many more DX cameras in 2012 than it has FX cameras, despite the introduction of the D4, D800 and D600. But of course it is the halo-model FX cameras that they want to show off about.
For me, the attraction of mirror-less is to have high quality in a small system, and in my experience the OM-D was the first camera I have used underwater that really cuts it. But the camera is particularly attractive to me because it achieves this image quality without having to have a big sensor. The D800 (and the D7000, actually) shows us that sensor technology has improved so much now that big pixels are no longer essential for excellent image quality.
If the OM-D had been DX I probably wouldn't have bought one. If it had been FX I definitely wouldn't. The attraction of the small, high quality sensor means small lenses and small dome ports. (Although I am still waiting on Nauticam for the housing). I want mirror-less to remain at a smaller sensor size than DX - I think the 2.0x crop of micro 4/3 is ideal.
Personally I am not motivated by travel weight. This is about having a small, high quality camera to reach places and more importantly angles that you couldn't with an FX SLR. A small camera when chasing pelagics (the OM-D does 9 frames a second when you get in position too), a tiny camera for getting macro shots on the sand, right at eye level, or a tiny dome for WAM shots with the fisheye etc etc.
I hope we don't see mirrorless race towards FX/full frame sensors. Same with compacts. Imagine having a camera the size of pack of cigarettes and having to use a 9" dome port on it to get good wide angle optics on FX underwater! Lets hope they realise small is good and mirror-less stays sub DX in format size.
Alex