I gave it some thought, but in the end I didn't think it would justify killing the warranty... if you're doing video work that requires better than mono audio, get an external digital recorder and sync the sound in post processing!
i got the D90 so i can have a second DSLR for my wife and i, and the video is a big plus. I HAVE good optics, and am very very happy w/ the D-Movie stuff, w/ exception of the built-in mic for situations like mounting the D90 on the outside of a moving car (did that last night).
I've been waiting for someone to try this, especially after seeing that the D400 didn't come out at PMA. I have no idea what it would take, or if it would even be possible....
I wouldn't even attempt to wire the AV Out to the audio input from mic... that's like open heart surgery for your D90, and one little slipup = fail. You would be wise to add a mini-jack or 1/4" input via the audio input bus IF you tamper with anything... rerouting circuits inside of the D90 may prove to be a fatal attempt!