This seems to be a hot topic lately. The other day i saw another post on a pi home security system that worked on the same lines as yours. Very cool though i will have to check it out.
I'm working on a very similar system with my raspi. but i use a 74HC4050 (high to low level shifter), on a seperate powered board, as protection for the gpio ports. So i can use a wide range of sensors (up to 12V supply voltage) without damaging my gpio ports. further I can choose to go to open collector outputs (like gertboard) for switching higher current devices as relais etc...
1-Wire sensors providing data about my heating system and room temperatures. so i can control it over my android phone from far away!
I hope that I can finish it soon, but there are always new ideas... ;-))
At http://www.projects.privateeyepi.com/home we have a very easy to setup and simple to use alarm system.
The system detects door swicthes or motion detectors and alerts you via email.
Create an account at www.privateeyepi.com and access your alarm system dashboard where you can control your alarm, configure locations and zones, view alerts and activity logs.
Follow the instructions on how to build the electronics and install and run the alarm.py file on your Raspberry Pi.
PrivateEyePi is free and created by Raspberry Pi enthusiasts for others to participate end enjoy.