The plan:
Sniff for authentication packets (the phones usually start trying to connect before they can actually reliably connect. So this packet can be sniffed out relatively early before they get there (particularly since the wifi antenna will probably be by/mounted to the door and the wifi router is on a shelf in the front room). When a user knocks it checks to see if someone has authenticated with the wifi in the last minute and if so, unlock the door. There would also be a backup secret knock that can be used if the person's phone is dead and hence can't connect to wifi.
Wouldn't the door unlock for ANYONE who knocked if your phone was on the network? Axe murderers, Ex girlfriends etc?
I dig the concept tho.