It's all rumour and speculation, but a picture paints a thousand internet links. This alleged HTC Touch HD is a fullscreen touch device with WVGA (800X480) resolution—essentially taking, say, the Touch Diamond's razor-sharp screen and making it longer. As another metric, if you think the iPhone looks nice, realise that screen runs in 480x320.
Other specs include 528MHz processor, 256MB RAM, 512MB, HSDPA, GPS and just 12mm of thickness. It's supposed to be "in stores for Santa." [Modaco Thanks everyone!]