It consists of a reed switch, an arduino and a router running OpenWrt. If somebody tries to open the door of my room, the magnet will move away from the switch closes the circuit. The arduino detects this signal and sends linux commands to the router using the serial port. These linux commands let the router send an email and as the internet connection could be broken it also writes to a log file.
After this was done a motion detector was added which tracks people entering my flat.
This project is considered to be a playground to get familiar with the arduino communicating with linux. If you think it's an ease to communicate with linux you're somehow wrong. Linux, or more precisely the console, doesn't only response with a command's output but also with the prompt string "john@mycomputer ~#" that you would see on a monitor. So if you send the command 'date' to the router, it doesn't respond with a text like "Tue Jul 26 09:26:12 CEST 2011" but something which looks like
john@mycomputer ~# date
Tue Jul 26 09:26:12 CEST 2011
I'll edit this article sooner than later with exact instructions of how to connect the arduino with your router and the arduino code will also be available very soon.