The current Android does not support tag emulation, so I must assume you will be using some kind of P2P protocol when talking to your door. If the phone sees the door as a passive unit, thats the same.
A key insight in your plan is that you should encrypt the communication to your door, and that the encryption key is really what matters. Any unique id you come up with can be faked, no matter where it comes from.
I'd put the encryption key somewhere online in a properly stored file. So then you are also covered once you loose your phone or it just breaks.
My approach would be to make a Google App Engine app which lets you sign in using your Android (email) account; i.e. an app which signs the user in using the built-in Android accounts. Put the encryption key there, possibly behind yet another password.