I suggest a call to Denon to find out if there are any known issues with this receiver with Sky, not that we encountered them with the less expensive model, or between the receiver and the TV. As I said, this whole HDMI thing is a bit of a minefield...
The workaround, if you have one of the Sky+HD boxes with component video output - not all models have it -, would be to run component from the Sky box to the Denon, and let the amp upconvert it and squirt it out via HDMI to the TV.
If it's any consolation, I had the same problem when the Sky engineer tried to replace my dying Thomson Sky+HD box with a Samsung the other week: as soon as it was connected to my Onkyo receiver via HDMI, it just locked up completely.
I ended up with an Amstrad box instead...