Take two guys, an idea, and a pizza box and what do you get? The Power Pizza, the "perfect disguise" for a mac powerbook (shown) or any other laptop. Unfortunately around here you'd merely be exchanging one sort of teenage felon for another (stoners get the munchies too!)
Personally, the very best deterrence I've found is simply throwing a towel, shirt, or jacket over valuables. When done correctly anything underneath is invisible. Because if a thief decides they want something in your car, there's not a car alarm in the world that's going to stop them.