http://www.robot-electronics.co.uk/shop/Thermal_Array_Sensors2086.htm
That's a pretty good Integrated sensor that you should be able to use on any i2c capeable breakout boards.
Kinda expensive. If I were you I'd put the sensor high up so the hooligans couldn't reach it.
good luck!

btw that's a decent camera for the price but it has limited range. I think if you added some flood IR lights it could pick up the IR light reflected back, because the limiting factor on the IR viewable is how much IR it can put out
http://www.superdroidrobots.com/shop/category.asp?catid=64

Also for turning on a camera reciever or what not you might want to check out sparkfun.com and look at thier wireless section.

This is what I'm imagineing. Gumstix sits idle untill it picks up some heat, turns the camera on then sends a signal to aregular computer that as video in and the film gets logged tivo style onto your host computer. there are some tricky parts in there but I think it's pretty doable.

--
        -Jesse W.