The Archos 5 is Android-based and connects to a TV via HDMI. All the usual Android apps are available on the screen, you can watch (and download) video, and you can connect a Bluetooth keyboard and mouse. So I think we are there, actually.

(The device then detaches from the TV dock and is a portable media player / Internet tablet. It was buggy at first, but I am liking it a lot now.)