Senors, software and such seem pretty error prone or otherwise defeatable. What about a "deadman's switch", a wrist strap connected to a short cable connected to a USB dongle. When the dongle is removed (laptop snatched) it would lock the laptop. The only way to defeat this would to somehow completely restrain someone from moving, and I can't see how you'd do that without seeing it coming. Even a tranquilizer takes a few seconds to kick in.