Airlock is an affordable $7.77 OS X application which installs a system pref onto your Mac, which in turn recognizes your iPhone or iPod Touch via Bluetooth to provide user-specific access based upon that digital handshake between phone and computer. When you leave your computer's perimeter with your iPhone or Touch, your computer locks itself from being used. Come back within Bluetooth range and it grants access. At least in theory.
Unfortunately we couldn't give the demo a full run due to a bug with OS X's Bluetooth implementation, which results in Airlock causing a connection loss with wireless Bluetooth keyboards like our own. It also failed at recognizing or finding my girlfriend's iPhone via the Bluetooth preference. Seems like there's a few bugs to ring out, but a free demo makes it an easy try before you buy investigation for those of you with suspicious minds in regards to roommates or (in)significant others.