According to this MSNBC article, Finnish scientists at VTT Technical Research Center of Finland have created a gizmo that detects changes in walking styles of whoever is holding the phone and if the walking style changes the phone will freeze to prevent unauthorized access.
If you want your phone to freeze to stop unauthorized access you could always just buy one of the Microsoft Windows-powered PDA phones… of course authorized access is also affected!
Seriously though, the technology sounds cool but what happens if you twist your ankle? Your phone will lock and you won’t be able to call for help!