My only concern about this is the fact that this system might be really easy to jam. I don't have a lot of experience with IR but wouldn't a lot of sun screw up the system or if someone walks in with a night vision camera/goggles? It may not be a big deal but something to think about.
Jamming won't occur if you shield the laser sensor properly. Also, you plan on having someone break in with nigh vision goggles? If you do then you might want to consider something more hefty than a missile launching toy
@biocow: Unless your doing this project for the sake of doing one with an arduino, I'd suggest just setting up a wide angle camera (or more than one cheap webcam) running some simple vision processing to detect "discrepancies" - i.e. something passing in front of the wall.