Follow up to adamz:
I recently bought the D90, but if I had it to do over again, I definitely would have gone with the D300 for the following reasons:
1. Ergonomics - I find myself fiddling with ISO & WB much more than I used to. I'd rather have the D300's dedicated buttons at the top rather than the D90's shared buttons on the rear. I'm a big proponent of the 1-button 1-function design philosophy. Lets hope apple never makes a camera. It will have a shutter button and thats it LOL.
2. Build Quality - I don't abuse the cameras very much, but the better weather sealing would be a huge plus, especially now that its started to rain again here in Texas.
3. Viewfinder - Full coverage is a good thing. I don't notice it much on the D90, which I think gives you 96% coverage, but it gives me fits on my D40 (90% coverage).
4. AF Fine tune - I never thought I needed this until I bought the 24-70 f2.8. This lens is razor sharp, but has a tendancy to miss the focus ever so slightly. I'd like to have the option of adjusting the AF on my camera to optimize it for each lens.
Unless the D400 is full frame, I'm not sure how they are going to make substantial improvement over the D300. Unless the kick the MP count up into the 20's, you won't notice a substantial difference in quality, and then your going to get noise on DX. I'd rather have the ISO's myself. Video... meh, I don't use it on the D90 much at all. I wouldn't turn my nose up at it, but I'd be disappointed if I waited and all I got was a video option.