Just now, I got bluetooth proximity detection working.
When I sit down at my laptop, the laptop unlocks. When I walk away, it locks.
It's detecting the presence of the cell phone in my pocket, via bluetooth.
If it's locked and I don't have my cell, I just log in normally.
It's Nipper-in-a-box. Or another case of Enterprise computing.
(Why would someone even want this? Oh, come on. Next question?)