Port knocking is a method of generating network authorisation for a source IP by making a sequence of TCP connections on particular port numbers to a host that is listening for them. Although usually used for opening firewalls and such, this alpha test implementation setting up dynamic DNS hostnames. The process for setting a dynamic hostname to a particular IP can be done as simply as a series of "telnet" connections to a sequence of port numbers, this can be easily scripted in pretty much any operating system. Linux even has a dedicated "knock" command to make it even easier.
Update 2020: this site was offline for a while due to a host change, but is back in 2020. There is now ipv6 support, if you connect via an ipv6 address it will create an AAAA record. If you connect from an ipv4 address it will create an A record. This is thanks to the experimental ipv6 knockd code from https://github.com/svalat/knock. There is also a version of the knock client that can do ipv4 or ipv6 there.
For further technical information here are the Wikipedia entries on Port Knocking and Dynamic DNS.
