I've already got an always on home automation server, an always on internet connection and several serial to relay devices which control my various systems around the house.
The idea here is simple, use my existing infrastructure to control a magnetic door release catch on my side gate.
The full plan is a simple web page + CGI which can be accessed on my iPhone (or other device with a browser)
Then just type in a secret PIN code, that will trigger the CGI code, write out a file on the webserver, which my home automation system will pick up, then parse and if all is well then switch on a relay which is connected to the door release catch.
Simple!!