Here's the set of photos I've gotten so far with the new camera (all photos zoomable via clicking on them, or try this alternative view). The Tamron 17-50 2.8 lens was already darned good on the D40; on this camera, it is absolutely splendid.
When I used to (try to) play electric guitar, I soon noticed the difference between a solid state amp and a tube amp. The D40 is the camera equivalent of a solid state amp. The D90 is pure tube. The image sensor can capture so much more visual tonality without either blowing out the highlights or having the shadows buried in noise. The available contrast, colors, and sharpness (assuming you work in RAW) are astonishing. The resulting photos look like something you can swim in.