I have now got my 2808 and some HDMI cables, and have tried two sources with different results.
Firstly, the PS3 appears to be able to output video over component and audio over HDMI. The only thing is the display on the 2808 always shows 'Multi Ch In' now regardless of what type of signal is coming through.
Secondly, my Sony DVD player does the exact opposite! Connecting both component and HDMI gives me sound (display shows DD/DPLII etc), but no picture. The picture only comes back when I deselect HDMI altogeher in the Denon input setup menu.
Does anyone have any idea what 'Multi Ch In' means?