The D90 is old tech now. The D7000 or the D5100 are the choices.
It's about the lenses. If you want a simple uncommitted relationship with Nikon, the 5100 has the same sensor as the more expensive 7000, but doesn't include the screw-drive for older AF "D" lenses - you can only use late-model "G" lenses.
The 7000 also allows you to configure the camera to recognize old manual focus AI and AIS lenses. These are cheap and extraordinary lenses, so the 7000 is the only crop camera choice for me.
The problem with DSLRs is that the makers are splitting the market with 2 paths of lenses, DX (crop frame) and FX (full frame). Can you say, "market segmentation?". You buy into one or the other. But, the best stuff is on the FX side.
If you have a commitment to nikon, you'll want to go with the better FX lenses and endure this period of crop sensor nonsense as best you can, or just bite the bullet and get a full frame body and get it over with.
I got the D90 for my wife and it is an extraordinary camera (D7000/5100 is said to be even better), but choosing lenses is a pain if you want the good stuff. It came with a wholly adequate tele zoom, which is fast in action. But, some of my old AI-s lenses strikingly better. We use it as the family cam - fast and furious, and it does its job very well.
Someday, this digital stuff will get worked out and I'd like to have the glass set up for that time. The plan is to get good FX lenses from now on, shoot them on the D90 cropper and someday follow them with a full frame body. Life is too short for crumby glass.
(I just want a little Nikon FE with a full-frame sensor... Which is the equivalent in RF to a M7 with a ff sensor, which is the M9. Who says Leica is behind?)
- Charlie