This is really cool. I've actually been working on something really similar.

Early last year, we built a a breadboard with a Teensy microcontroller (instead of an Arduino) to decode signals from off-the-shelf door and window sensors. At the time, we had to reverse engineer the protocol using a oscilloscope. :) The breadboard was tethered to an Android device, which was the brain of the system (and this let us do other cool stuff, like video, networking etc).

We're much further along now (with working prototype units). If you're interesting in this kind of thing, we ARE hiring. :) (http://elarm.com)