DryFire - how to find a spot on the wall

Hi,
I've been thinking about doing something smiliar to http://dryfire.ie (clay target simulator) - much simpler but the idea is to project a spot on the wall and then "fire" either another laser or ultrasound. The problem is - how to find where that "fired" shot was on the wall (ie. did it hit the projected target). Any ideas...? Some sort of triangulation (where the device projects laser spot on the wall and sends ultrasonic pulse and then another pulse from my "gun"...)?
Any idea would be appreciated...

have a look at this discussion - Impact location, using four sound sensors. - Sensors - Arduino Forum - might be interesting (or scary;)