Z-Wave, the wireless mesh protocol behind most of these systems, is an open protocol. That means you can build yourself an a la carte system. Not just for security, either, but total home automation. You can even buy Z-Wave USB dongles that work with Linux. Of course you'll need software as well. Or you can pick up an embedded device such as Vera Lite. If you're okay with a small monthly fee, Nexia (formerly Schlage LiNK) is Z-Wave based.