Found this from Initial State via Life Hacker, Know Who’s at the Office with the Raspberry Pi, http://blog.initialstate.com/know-whos-at-the-office-with-the-raspberry-pi/#!prettyPhoto. Seems like it would make a good replacement or augment the ability to detect presence. I know in my case, I connect to my WiFi before I enter the house.
There are a number of community members already doing something similar. This type of option is discussed in the presence FAQ.