I've had similar problems using XBMP lately. I have the september 03' CVS build. My problem is that sometimes my divx files will play flawlessly, and sometimes only the audio will play and the screen is just black. Even after tinkering with the video settings, it still gives it trouble. These are divx files that I have encoded myself using divx 5.02. Usually whenever I reboot my xbox, they play just fine again though.
This is probably something that will be fixed in a newer cvs build. I'm not sure how many versions of divx the current build fully supports. Sometimes it likes it, sometimes it doesn't. It's weird.