I was looking into doing some sort of visualiser the other day, although not with lasers, but I didn't find any good info on detecting/processing frequencies with Arduino. My idea was to have a series of band pass filters connected to analog inputs but I am no expert on audio electronics (yet), how would you go about implementing this? Could it be done with less circuitry?
Afraid I haven't looked into it either 
I was probably going to get my hands on one of those audio processing shieldy thingies.
Mowcius