Exact same thing as you. When all devices connected directly to the HDTV as opposed to through the Denon they map perfectly at 1:1. This with my Panasonic VT30. I've tried changing HDMI cables and using different ports on the TV with no success. I have the problem with all my components (PS3, Apple TV, HTPC, XBox 360) so in my mind the problem is the Denon 1912 NOT allowing 1:1.
Also observed that when turning on overscan (normally off) or using one of the aspect mode zoom levels on the TV all pixels are being used. However, when using straight 16:9/1:1 mapping on the TV with overscan off, the farthest left column or 2 columns of pixels are not being used.
Did you find a solution?