I've used Bluetooth Lock
http://btprox.sourceforge.net/ It's free and open source.
The problem I had was range. If I'm at a conference and leave my table I would have to leave the room before it locked the screen. Same thing at work. In a cubicle farm, I would have to go 20 or 30 feet away before the lock activated. Reactivation was about the same. If I approach, it was frequently open before I got back to it. (Yes, I know it works because I could see the screen :-)