There's floodlights available at http://www.smarthome.com/4080xt.html which will send an alarm when they activate using your power lines. They also sell a X10 motion detector without the lights for about $25. It seems that this would be the easiest way to install what you want without stringing a lot of cable. Interfacing a PIC to X10 seems fairly easy although I've never done it.
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