I upgraded from d90 to D800. I still have d90 and used it side by side with d800 last weekend. I had 70-200mm vr2 lens on d90 and 14-24 on d800. Both cameras took great pictures. Due to the crop vs fx factors i really don't see d90 now as useless. Yes, d800 has a 16mp equivalent crop so its better resolution crop than sensor on d90, but there is still allot to be said about flipping d90 to full auto and just machine gunning for long distance shots.
D800 is a great camera in terms of image and build quality (left focusing point does not bother me), but i have no intention of throwing d90 away, now that i have d800. After using d800 for several weeks i find a d90 to be a nice "point-and-shoot" addition as compared to d800. Although i am thinking of replacing D90 with D7100 if/when it comes out. I know that d800 can be shot in crop mode and be equivalent to d7000, but shooting fx in crop just feels wrong to me.