For what it's worth, I have some home automation setup that checks for my iPhone's presence on wifi (my phone gets on wifi every ~10 minutes on battery, and holds a connection when plugged in). I preordered a Lockitron, and plan to use the API in my "welcome home" script trigger to unlock the door.

I'm not sure how easily you could implement this sort of detection on Lockitron's end, though. I use a static DHCP lease on my router and simply ping the phone.

Just thinking out loud here! Wifi presence seems super useful from my limited experience, and I'm not sure why more tech things aren't using it.