I found this on Denon site:
http://ca.denon.com/2082.asp
Unfortunately, it basically says that it is not the receiver's fault (what a surprise!)
Quote:
"This is not a fault of the receiver. Recently set top box manufacturers have become aware of this issue. It has been found that the U-code information appears to be the problem which is part of Annex A of the repeater conformity specifications of the HDMI protocol. Some set top receiver manufacturers have already implemented firmware changes to their units while others are in the process of updating their units. If a set top receiver being used is having problems passing HDMI video signal through an A/V receiver, contact the manufacturer of the set top receiver to see if an update is available."
So I'm taking this to mean that my receiver is very picky about the HDMI protocol, and the ATV isn't quite conforming to it. Maybe other brands of receivers are passing the signal?
I suspect I probably won't have much choice for now but to either connect the ATV to the Denon using component cables and an optical cable for 5.1, or use HMDI directly to the TV for video and an optical cable for 5.1 audio.