This is a great idea!

The Plexidor Pet Door is an interesting alternative. After installing the mini door, only pets with the compatible RFID chips attached to their collars can enter. The door opens when it detects a nearby chip, which the company assures is waterproof and shockproof.

Read full story here:

http://latimesblogs.latimes.com/unleashed/2009/11/dog-door.html