To address the original question...
I'm in the same position, I have been using a Canon A640 (which flooded on my last dive in the Philippines a few weeks back) and, whilst I'm very happy with the images I took with it, I do want the superior image quality that only a larger CCD - like the ones found in DSLR's, can produce.
I played with a canon 40D in a camera shop last week, and the live view is nice, but as someone pointed out, it doesn't let you use AF whilst in live view. Very annoying. Kind of defeats the purpose for underwater use, the Nikon D300 will probably have similar problem. I am looking at these 2 cameras, as I would rather go with live view than spend an extra $1000 on a viewfinder. I am one of these blind people...
Now that I know more about live view and the way the new Cameras work, I am actually tempted to drop myself back to a Nikon D80 or Canon 400D, and spend the $$$ I save on glass.
Whilst it is true that DSLR's are going to cost you more in every way, WELL composed shots, that are correctly lit etc WILL look far better. Just as the shots on my A640 with an Inon strobe look better than my pics on my old Pentax S5i with internal strobe only.
The other thing - about being stuck with one lens - sure, you are stuck with macro or wide angle for the whole dive, but, in general, I KNOW what sort of dive I am going on to begin with.
I don't go to Macro heaven locations like Cabilao expecting to see a Whale Shark, and when I dive Monad Shoal in Malapascua, or the Yongala wreck on the Great Barrier Reef, a wide angle is a pretty safe bet. There are always going to be missed photo ops - if I am set for macro with my A640 and I am in need of a wide angle shot - by the time I set my exposure, aperture, strobe power etc. to what I want, I will most likely have missed the shot anyway...
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