I have skipped several iterations of upgrades, because the camera I was shooting worked fine for my style of photography.
I decided a while back, for some future work I want to do, that the D600 was the right camera, right time. I was waiting for the special pricing, (bought one this morning), figuring it would be the camera that would replace my D200,(which will most likely get converted to IR now). The D600 will be my main camera for years now.
Point is, if the D90 still suits you, why upgrade? You could buy some nice lenses for that sort of money, perhaps a high end printer, or any of a number of things that you might find more useful to your photography.
Just a perspective.
"Extraordinary claims require extraordinary proof " - Carl Sagan