These are all nice ideas, thank you. I was also thinking about an RFID tag idea, where the (house) doors would unlock if you just went near them. The GPS idea is cunning.
Probably a good security system could combine all sorts of sensor input, eg. where you are, the time of day, movement, door positions, day of the week. So for example, if movement is detected in the house on a Tuesday afternoon when you are normally out, and the iPhone GPS reports that you are in fact out, then that could be suspicious.
what is the purpose of it? To remind you if you left the door open?
Basically yes. And in fact, since you can accidentally press the button it isn't so much a reminder as information. The way the door faces, you can't easily tell from inside the house if the door is open or not.