The D90 has always provided plenty of performance. However I have begun to use my AI and AIs glass more and more with it and it sort of sucks not to have real metering..... that's what got me looking at newer bodies which led me to consider the D600. However I am considering more contemporary DX cams now as a result of this thread.
I think it makes a lot of sense to at least try one of the newer DX bodies that will meter AI lenses first, since you'd have to redo your lens kit if you jumped to full frame. If it really doesn't work for your needs, swapping up later won't leave you too badly out of pocket.
The 7000 is still a very capable camera, and the newer version getting strong reviews. (Having used mine in serious winter conditions recently - strapped to my back on a snowmobile on a frozen lake, operating with mittens - it has some legs). For the money, and for many, a second body might make (more) sense compared to a jump in format.
As for the discussion about wide angle lenses, I recently purchased the 10-24mm DX lens. For the money, it's about the difference in cost between the d7000/d7100 and the D600 (not cheap but not outrageous), by most accounts a strong lens, not too large and would be hard (and expensive!) to acquire a lens with comparable characteristics on FX.
Most importantly, it gives me the wide-angle coverage I'd really been lacking without too much expense or starting over. Even though I don't have that many DX lenses, my overall lens 'kit' fits pretty well with a DX body, and moving to FX would present some quandaries.
That said, I still lust for the full-frame and understand it makes sense for many (and I doubt many who do move up regret it). But for now, when I seriously want to play in FX-land, I get out the F100.
