Personally, I think DX is here to stay and the reason is all about the economics of chip fabrication. The bottom line is that the FX chip is more than twice the size of the DX sensor and I'm guessing that they cost 3 to 4 times more to produce. There are a hand full of us that are willing to buck up a couple grand for a camera, but it isn't people like that who drive the market. I'd think that the real bread and butter of the market is for cameras like the D40x and the Rebel XTi and I'm not convinced that these camera's will ever see full frame.
I think the chapter for DX sensors in pro level DSLR's is already over and I doubt that FX will ever make it's way into consumer cameras. I also believe that both DX and FX have a place in prosumer level camera. I believe that in the future we'll continue to see divergent evolution of the prosumer level cameras. Camera's at price points in the Canon 40D to Nikon D300 range will be DX while cameras in the Canon 5D to Nikon D700 range will be FX. Isn't there is enough room for both markets?
There has been some discussion on the forum about whether or not DX is obsolete and if it's risky to buy lenses designed for DX sized sensors. Personally, I don't see an end in it's future. 35mm evolved in the presence of medium format.... I don't see any reason why DX could, would or should go the way of the Dodo bird. Whether or a DX lens is a safe investment or not depends on which camp you see yourself in the future.
Cheers,
Mike