I have a Comcast Motorola HD-DVR (HDMI) and an Onkyo receiver too and if I try to connect the two with a HDMI cable the Comcast HD-DVR has trouble handshaking with my Onkyo, and displays a message saying that there is a HDCP handshake issue. Basically the cable box reads the receiver as a possible illegal recording device and just will not work with it.
Your best bet is to run an HDMI cable directly to your HDTV and a digital audio optical cable from your DVR to your Onkyo AVR. The Motorola DVRs that Comcast uses are notorious for not receiving firmware updates that would fix this problem.
Ok, so I've used Comcast's DVR's with no serious complaints, until recently. I recently purchased a Onkyo home theater system. It seems that Comcast's DVR's do not want to work through a A/V reciever. I'm using HDMI cables to hook everything up. It took me awhile to figure out exactly what's happening and to this point Comcast's phone technicians and the two they sent to my house how been little help. The first time I called they suggested that I bring in my box and get a new one. I did and got the newest motorola box they offer. (sorry it's late and I don't want to go downstairs to see the model number...shouldn't be important because it happened on the old one too)
So basically the DVR freezes and becomes unresponsive shortly after turning off my AV reciever. "dvi" flashes every second or so for split second on the display screen. It appears like it's trying to determine what cable or signal or whatever it's supposed to send out if that makes sense. Let me say that I'm definitely not to knowledgeable on this kind of stuff but it seems that this constant "dvi" seeking is causing it to freeze. Theoretically the DVR should simply be SENDING video and audio signals to my receiver but it seems to know when it's on and off.
If, after done watching tv, I turn the tv and reciever off (no-matter whether the DVR is left on or off) the DVR becomes unresponsive and I am forced to unplug the power cord and plug it back in. So basically unless I want to leave my reciever on all the time, I'm going to have to do this with the power cord every time. Not only that, because it freezes, any programs that are preset to record will not be recorded.
The only thing I've been told that I can do to fix the problem is to use component cables instead of an hdmi. I really really want to avoid this if possible because that takes a lot cable I gotta run up to my tv and junk blah blah blah. Anyway if anyone could help me I would really really appreciate it.