A few months ago i "upgraded" my D90 to an used D700 (real good price with only 8K clicks), but not because i needed but because i want/could. I'm keeping the D90 as my second body and i'm using it in a week basis, and planning to keep it "forever". Though my D700 is a new league, my D90 is much more of a camera than i am a photographer. When i want to travel light, and i travel a lot (being a airline pilot), i only take my D90 with my DX lenses 16-85 (or 10-24) and 35 1.8. Never regret to have that combo while traveling.
Nikonians!!! My best investment made after my camera!!!