Since the Wiimote can track sources of infrared (IR) light, you can track pens that have an IR led in the tip. By pointing a wiimote at a projection screen or LCD display, you can create very low-cost interactive whiteboards or tablet displays. Since the Wiimote can track upto 4 points, multiple pens can be used.
I'm am sooo building this.
http://www.cs.cmu.edu/~johnny/projects/wii/ (second entry)