Using your iPhone, Bluetooth wireless technology, and a smidgen of pixie dust, Airlock determines whether you're near your computer. When you leave the room - poof! - your Mac locks itself. "And when I come back?" You guessed it: your Mac unlocks. You can also customize Airlock to perform specific actions as you come and go - have your computer talk to you, log-in or out of iChat, walk the dog, and such.
Looks great, and reminds me a lot of "Sony Ericcson Clicker," which I wrote about eight years ago: "It is the [ability to use AppleScript to let your presence (or absence) control anything you like] that excite[s] me most. The software essentially allows your mobile phone to function as a ‘proximity sensor.'"
Update: My friend, Richard Felix, tells me this software has been abandoned.