What probably would be better is if a device is integrated in a PCI/TypeII slot and remained inconspicuous to a thief, but simply received a beacon signal from another device somewhere on the same premises. If the installed device were to be moved out of a certain area the device would stop detecting the beacon's signal and simply destroy the host its installed upon.
Something could be adapted using the 802.11b home wireless LAN technology. If the owner, or someone on the owner's behalf, needs to move the computer (PC, laptop, Mac) from the premises they simply shutdown the device in a proper manner. The device can be passive and run on a rechargable battery (laptop), or trickle power (PC & Mac). The associated device can be simple, hidden anywhere in the home, and offer a 'safety' field for one or more computing devices.
If the device is needed for travel (such as for business travel), simply bring both components and hide the beacon device under a bed, in a closet, or within a range of a parked car with the beacon running on a DC-adapter from the 12V source. this would also work for laptops sitting in vehicles, too.
"He who can destroy a thing, controls a thing," Paul Atreides, Dune