To me a camera is just a camera; a tool. The fact that a new camera came out doesn't mean that it's better than what I currently own. I'm still able to do what I was doing before. What a new camera can do however is open up new possibilities (I can shooter quicker, longer, easier, etc.). This is the primary reason why I haven't gone beyond the D2X/D200 combo (looking to seeing what the D3X looks like).
As for D200 vs. D90, it's gonna depend on what you're interested in.
Even though the D90 has the same sensor as the D300, it does not have the same focusing motor as the D300; it has the same focusing motor as the D200. So autofocusing wise, it's not gonna matter which one you go with. Also, the D200 is faster with bursting still vs. the D90. Where the D90 will perform better is in the areas of noise. The other area where no other Nikon DSLR will even compete in is the ability to shoot a movie (to a limited extent; first gen product - no autofocus ability while doing so....). The D90 also uses SD cards instead of CF. Memory cards are pretty cheap nowadays, so it isn't really an issue other than the inconvenience of selling/purchasing cards.
As to whether it is better than my D200, I can say that will be in the levels of noise and the movie mode. Other than that, the D200 will be better. It's also gonna depend on when you need the camera. If you need it now, then the D200/D300/D80 are your current options; if you can wait until say November/December, then the D90 is another viable candidate. Upgrading from a D80 to a D90 will not be as significant as upgrading from the D80 to the D200 IMO (unless the production model does more than what's stated on papers).