I almost bought a D90 but I decided against it. Didn't like the features. I don't like mixing a still camera with a video camera. I have a Nikon D300, a D50 (which is a fantastic camera) and an old D1. Love them all. Would be afraid the D90 would spoil my feelings about the other Nikons I have.
I too have a Nikon D300 like Black Dog, bought it from my brother that found out it was way more complicated than he wanted, he bought a D90 and loves it. I shot with one in a dark building the other day when a friend brought his to work, perfect exposures in situations that shouldn't have been so easy. Much lighter than the D300 and a lot of fun, you can concentrate on your composition and just shoot. Great choice for your first serious camera, I've talked to a number of people that have them and love them.