I actually considered doing that with 8 by 8 cheap lasers from dealextreme, until i started thinking about how to align them precisely enough to be usefull
Yeah, not easy...
Go buy yourself a large quartz crystal from one of those gift shops that sell cool rocks and stuff. Place it in a good location in your living room. Mount all the lasers in random places on the ceiling, pointing at the crystal. Modulate the lasers with your Arduino to put some life into it. You could use sound, motion, distance, temperature and/or light as input info. Put a bright white background behind it to catch the scatter of the lasers for more effect.
I like it!
Mowcius