My wireless router is reserving a certain IP address for my phone by MAC address.
It's also not broadcasting is SSID and is password protected with a WPA2-PSK key.
The Arduino Yun, using its Bridge library, can execute shell command PING and periodically check to see if my phone is on the network.
I retrofit my deadbolt with a hobby servo to be controlled via the Arduino when my phone is detected on the network or when an inside "Door Unlock" button is pressed.
A door sensor will hold the lock until the door has been opened and closed again. it also prevents the lock from locking the door open and can give me feed back on when the door is opened without the servo being actuated. (i.e. someone used the key)