It sounds like quite a good idea, I don't see why it would not work. I just recently had an idea to have cheap lasers (dollar store ones) at doors and if someone crossed the beam, I would turn one of those USB missile launchers and shoot at the door.
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.
I haven't gotten my Arduino yet so I don't know the detail behind this, my firstish project is going to be a universal remote or a TVbGone clone.