Dynamic DNS Updates with BIND and an Airport Extreme

My home network has an Apple Airport Extreme as the first stop from my cable modem, which of course provides dynamic addressing, and I run my own BIND DNS server on the Internet. I wanted a way to query the Airport Extreme for its external IP address and dynamically update a host entry on my Internet DNS server so that I could access my home network while away. I used to subscribe to a service called DynDNS (there are others as well) but I prefer to be in control for this scenario. The BIND setup is easy: I just created a TSIG key and allowed the zone that contains my home network's host address to accept dynamic updates using that key. There are several examples of this on the web.