I have the 6416, and Comcast upgraded our firmware about 4 days before my HDMI cables arrived.
Plugged it in through my Onkyo TX-SR604 HDMI Port 1, and no picture. Spent about an hour on the phone with the comcast technician. She never solved the issue, but she was very pleasant and did know about the HDCP issues, and she tried to troubleshoot that. When we gave up she took my information and told me that she would have another technician, who "was very versed in this stuff" contact me when he got off of his next call. Must have been a long call, because a week and a half later I still haven't heard from him.
Called Onkyo, and they suggested performing a clear on my receiver, and configuring the HDMI ports again. I did this, and it didn't seem to work. Then I decided to try moving the cable over to HDMI port 2, and viola!! It worked. HDMI Port 1 wants to automatically configure to the DVD input when you turn on HDMI switching, so I'm wondering if Onkyo won't allow the connection for non-DVD sources through HDMI 1. Sounds far fetched to me, and when I get a few minutes (my cable box is in the basement, and my tv is in the family room, so changing sources and testing requires a lot of trips up and down the stairs) I'm going to switch back to HDMI 1 with the Comcast box to see if I still get a picture.