Here is the supposedly leaked Xbox dash update which should be coming in the Fall. According to the list, divx support has been added along with a ton of other features.
* Compatibility with 3rd party wireless controllers
* Compatibility with messenger pad
* Xbox Live Vision firmware upgrade, sharper image filtering
* Many more Vision Cam settings
* New screensaver settings - when away from Xbox 360, screensaver will play for optional time, before screen goes dark
* New Xbox Live Arcade blade - where you can download, play and learn about Xbox Live Arcade.....