I hacked an X10 PIR motion sensor since I had it around and useless since I don't use X10 anymore. To do that I just used a meter to determine what changed when it clicked and soldered to that.
Also a broken stereo had some really high quality rotary encoders and button matrix setups with nice wires - little to no soldering needed. I posted details along with the pictures and code for the encoder, button matrix and the discovery phase in the exibitionist area - but may be useful here as well - so here is a link.
http://arduino.cc/forum/index.php/topic,67382.0.html
I have yet to hack another but will for sure pull a stereo like this before the "clam truck" eats it.