The problem with bankers and lawyers is that they look and smell too much like real people, so it is very difficult to differentiate between them and us.

RFID tags are one way to go for detecting friends from foe, but if you really want to get into it, you can use biometric stuff. There are some open source programs availale out there, and basically all you do is point your camera at the door or cat flap in question and if the computer recognises the shape of the face (or hind quarter if they are on the way out) then they are one of the family. Drawbacks are that it will have to be PC based, but there is less to actually build. The other advantage is that there are several programs out there that will send an SMS or MMS from your computer to a designated mobile. BUT, it's really all out there and ready made for you, and that is just BOOOOOOORRRRRRIIIIINNNNNGGGGG