Also being the owner of a D90 and D7000, and having used a D300s quite a bit, here is how I see things:
The D7000 is superior to my D90 in every way possible. How superior? I don't think most people should ditch their D90 and rush to buy a D7000 unless you need another stop of light and already have fast glass, or unless you need a faster and more accurate autofocus system, or unless you need some weathersealing. To me, those are the three biggies and they may or may not mean anything to you.
Now against the D300s? Well since the D90 and D300s have the same sensor, and in most tests the D90 even has a slight edge in low light capabilities, the only reason I can possibly see to take the D300s over a D7000 is that the D300s has more FPS and is better weather sealed. If you need 2 more FPS get the D300s, or if you are planning on shooting in the pouring rain without any protection for your gear (which I think is stupid anyway). To 90% of the people using a D300s 2 more FPS will not make any difference. If you needed FPS that dang bad you would be shooting a D3 anyway.
I tend to agree with most of what Ken says about the D7000, it is an incredible little machine. For the money, I think it is probably the best out there right now. That, of course will change soon enough
One huge advantage is that it is driving the price of D90s down and they are still awesome little cameras. If you have a lesser camera and want to upgrade but don't need the features of the D7000, now is an excellent time to get a great deal on a D90.
Allan