I'm wondering how well this concept would fly.
On the one hand, you could setup follow me quite easily, and nicely.
On the other hand, I want it to unlock my door when I get close to it, and lock when I go away but there's too many 'what ifs' in that scenario for my taste.
But I like the idea of RFID'ing my cats collar and unlocking the pet door for her, that would be kinda cool in that geeky way I love.