I don't know if anyone else has seen this, but I just came across this application in a recent article in Information Week.

It's a pen-based app for a tablet PC that can associate mathematical equations with sketches, and then animates the sketch to demonstrate the results of the calculation.

There's a WMV video presentation you can download further down on the page in the Publications section:

http://www.cs.brown.edu/~jjl/mathpad/mathpad.html

I think it's pretty darn cool.

Laterness,
Jon