In late 2004 I paid $5800 USD for one of the first Nikon D2x cameras in the USA. 14 months later the D2Xs came out and "obsoleted" my camera. However, the D2x today, almost 6 years later, still makes great images.
I learned something from the pain of having my expensive DSLR made obsolete. It just doesn't matter. It takes several years for DSLRs to really progress to the point that technology truly obsoletes a camera. I am still shooting from time to time with my old D100 from 2002, and it still takes great pictures.
I have a D100, D70, D40, D40x, D50, D90, D3000, D5000, D200, D300, D300s, and D2x. Everything from the D90 to the D2x takes virtually indistinguishable images at normal ISOs. Only when I go above 800 ISO can my D300s outperform the D2x, for instance. The screen on the D90 and D300s is so luxurious that it makes me want to use the cameras instead of the poor old D2x. It's not so much the image quality that is at question. It is the feature set on the newer cameras.
If you see a camera you like now, buy it and use it. Don't wait for the next greatest thing out. Your camera will be obsolete by this crazy world's standards within a year of buying it. It is a game the marketers have been using since early computer days. Make 'em want the latest thing. Upgrade, upgrade, upgrade! Upgrade your cellphone, car, camera, house, spouse. Upgrade!
Of course, when NAS comes a callin', I start buying too. Buy the D90 and never look back. It and the D5000 are some of the best hardware Nikon has ever made for the price (in my very humble opinion).