I love the do-it-yourself attitude of some e-learning tech support guys! Marko Puusaar just showed to us here in the e-university conference what he hacked together based on http://johnnylee.net/projects/wii.
In the picture you can see the touch screen for less than 50€. It took him about 15min, and that is with all the explanations of parts included!!
The pieces needed are: a Wii remote control, which will work through blue-tooth with your computer, a bit of software (there are pay versions, or free ones for educational use), and a Infra-red pen, which he did himself to look like a normal pen.