Ok, I tried a dvd on the 360 last night. It all seemed to work ok. If there was any delay, it was very minor. Btw, are you using your 360 on a DLP tv? I know the samsungs were notorious for having a bad delay if they were scaling from a resolution other than the native resolution. Like 1 to 2 seconds is very common problem. So, the xbox 360 runs in 720p for all your gaming, and the sound matches, since the tv isn't scaling. DVDs run at 480p, so the tv takes a long time to scale the video and your sound running to the receiver processes the signal instantly, so you get the bad delay.
Let me know what TV you have, and I'll see if I can find if it is one of the problem units for video lag.
Later.