Airlock: " Airlock unlocks your Mac when you approach it, and locks it again when you walk away, using the magic of Bluetooth to detect your proximity by looking for your iPhone (or later-gen iPod touch). Always assuming you don't forget your phone at your desk, of course." [and there's a way out of that]
http://www.macworld.com/article/144650/2009/12/airlock.html
I haven't yet tried it out, but might. There is a caution at the Airlock site regarding wireless keyboards and mice, though]
