I don't know if this is helpful, but here's what I learned last week. I had a Motorola 6400 non-DVR box hooked up with a DVI-HDMI cable plus a separate sound connection. The box had only DVI output, not HDMI. Suddenly while watching, it began to behave poorly - picture full of artifacts, guide not displaying properly, time reset to midnight. Soon after, no picture at all was displayed, though the sound continued.
Comcast service technician who arrived the next day said that the current firmware upgrade had broken the support for DVI cables. He provided and hooked up component cables and the video immediately appeared. A subsequent call to Comcast customer service confirmed they are aware of this problem and that Motorola was working on a solution - but no date for a fix could be provided yet.
Being unhappy with the picture quality with component cables, I returned the box to Comcast and obtained a DVR box with HDMI output - Motorola 3616 (I think). After hooking up with HDMI cable, I am now pleased with picture quality (as I was with the DVI-HDMI cable). Also love the DVR capability.
Bottom line - the current problem is with DVI cable support, not HDMI. Who knows when it will be resolved. . .