To be honest, I doubt the results would be that good enough for drawing. As he said, the resolution isn't as good as an actual interactive whiteboard. Wacom tablets have an even greater sensitivity than a whiteboard. I've actually used a interactive whiteboard to paint a picture (in photoshop) and the sensitivity wasn't all that great, but it could just be that it was a cheap interactive whiteboard.

However, if it's the pen that's lacking, I'm sure it wouldn't be all that difficult to make one. Infrared LEDs are pretty cheap, about 10p ($0.05) each.

The idea is really interesting though, if I get the time I might look into it...