I think that would be the real winner - locks when you walk away, unlocks when you return. So simple I'm surprised it doesn't already exist.
It does but most of the implementations I have tried (Including multiple PAID ones in the app store) don't work well at all. I would settle for locking when I walk away then tap to unlock when I get back.
Or that Apple hasn't built it into iOS and OS X by now.
This. At this rate, Microsoft will do it first to sell more phones.
BlueProximity does. It's a Linux tool, Mac OS X support is pending.