I know there is nothing more obnoxious than a once resistant convert, so please take what I say with a pound of salt.
I own both, and, even though I didn't want to acknowledge this for the longest time, about the ONLY good reason to choose the D90 over the D80 is cost, unless of course one already has a D80 and is content with it.
It was only when I got my hands on some real life high ISO NEFs (never mind those artificial imaging-resource samples), converted them according to my prefered workflow, and took a long hard look at them that I realized I really wanted to harness the High ISO excellence of the D90.
I like high ISO/low light stuff, and I have spent months with my D80 trying to play to its features and limitations. I have tried most NR software and have gotten adept at it. I have spent hours of tweaking in PSE, tweaking here, selective sharpening there, blurring over there. I have dropped eyedroppers into shadows in an effort to selective reduce the effects of a blown blue channel. I have fiddled with film grain plug-ins in an effort to mask with a more pleasant grain effect the less pleasant effect of digital grain.
Given all of this effort, with at best inconsistent results, you can appreciate that I saw the high ISO performance of the D90 as a revelation.
Despite the DPreview, imaging-resource samples, and the claims of so-called pundits here who have never actually used the camera to any great degree, the D90 is so much more than just and expensive D80 with some desireable extra features.
I am so glad I put my stubborn hubris aside and caved in and splurged. I will continue to take my usual lot of bad pictures, but I will relish being able to do so with more flexibility and less noise and colour distortion. The D80 is a great camera, and at present liquidation prices is an amazing deal, but for the extra money the D90 is in a whole different league.
Les