Is there any device that allows an HD source to be displayed on the desktop on a Mac or even a PC? As far as I know there's no such product available. Basically a device that has DVI or HDMI as an input and simply displays it on the desktop.
Not to my knowledge. It would be great if someone would make one - the closest I've seen are the HDTV importers, like the Elgato EyeTV 500 and Miglia TV Mini HD, but nothing that imports from DVI.