I just purchased the Onkyo 805 and I subscribe to Comcast HD. The 805 beauty nicely solved my chronic lip synch problem I was experiencing with my Denon 1803. In fact, I hardly take the receiver's lip synch adjustment off 0ms.
Perhaps the difference is my Comcast box (Scientific America) doesn't have hdmi out. So I'm using component out from the cable box, routed through the 805, and then component out to my Samsung dlp. I experimented with component in/hdmi out, and didn't hear/see any advantage (I'm looking forward to purchasing a blue-ray player to take advantage of hdmi in/out!).
I can't duplicate your experience, so I wouldn't rule out a defective unit ... either the receiver or the Comcast box.