Just thought I'd mention this project i did, incase others were keen.
I developed a little box at home that has 3 hooks protruding horizontally out of it. Each hook is soldered to a micro switch (hidden in the box) and the box is mountd to a wall. When a set of keys are hung on the hook, its weight pulls the hook down and activates the micro switch, which i use a SECU16 connected to an Ocelot (though you could use cheaper alternatives, like cutting off an MS13's senset detector and using those leads, or connect up some other sensor interface), to notify my Home Automation system that the person is home. When the keys are removed, the micro switch auto flicks back up. It works beautifully.
Although not a prerequisite, I'm also using the
MyServant Software (
www.futuretouch.com.au/index-3.asp) to track who's home and who's away. It shows an image of each occupant for the home, and an indcation of whether they're home or away, with status indicators, and the last time they arrived/departed. It also knows that when the last person takes their keys, auto close all blinds, shut off all lights, divert phone, etc (this is achieved sing HAL). And MyServant also farewells & greets each occupant. Oh, and since it also has random-personality responses; if you take/hang your keys too often, the PC gets abrupt and changes its farewells/greetings to a different set of responses!
If people want more explanation on how to build the box, just ask.