I saw a technology demo a long time ago, I think from Philips, that used a camera pointed at a whiteboard. Aside from the typical "take a snapshot of the board and fax/email it to me", the board could recognize and execute various actions by writing words and symbols on the board and drawing a circle around them. (e.g. "75+circle = set the room temp to 75 degrees")

This was way before cheap vision systems were around. I bet someone could hack something together today with a cheap Linux SoC, webcam, and OpenCV.

Combined with a video projector it could do even more. But then again, I don't want a noisy projector running all the time in my kitchen.