I have thought of this myself. If I made a small 3 wheeled robot that wandered around the house, it would be nice if it could plug itself in.
I was thinking of some sort of platform that the BOT would have to drive into. The platform would have a set of electrical contacts that could detect the BOT and start the charging process. To help the BOT find the platform, perhaps an IR LED on the platform that had a specific signal. The BOT would have an IR receiver that looked for that signal in the house and if found, went directly towards it.