I previously wrote that I would not use my Xbox 360 as a DVD player due to some motion artifacts I could see.
I did a test again after loading the the Xbox 360 Spring Update. One of the features in the list is improved video quality for DVD playback.
It looks like this update did the trick. I don't see any of the motion artifacts any more. I watched the credits of a movie and they slid up the screen smooth this time (before the Spring Update, the credits would have a slight jerk every second or so). So *now* I will use my Xbox 360 for watching DVD movies.