Today [ 16/01/2010 ] I found this "passive" sensor for an alarm system . Coincidentally it was also $9 and came with a 9V Duracell battery worth about $7 .


Since it doesn't switch any sirens or anything it may be a little more tricky to find a supply to connect a power outlet to a remote . Then again it does have a row of switches to change the wireless signal output . 10 switches each with two combinations = 2x2x2x2x2x2x2x2x2x2 = 1024 possibilities - maybe one of them will be the right signal to trigger the camera ;) .
The instruction manual states that it would last one year triggering 100 times a day and activates for 55 seconds which means you could have the camera in continuous autofocus mode and continuous shooting mode and it should have time to focus on a moving object and take a few shots in that 55 seconds .