This is truly awesome. The folks at Graffiti Research Labs created a rig that facilitates painting with light on the side of a building. It uses a high lumens projector to project the light, a green laser pointer to do the writing, and a security/astrononmy camera to detect where the green laser was pointed. They have made all the code available under the GPL.

Here is video of them using the rig via YouTube:

Higher quality quicktime video can be found here.