Q: Why would you want to run your own dynamic DNS service?
A: Because you have your own domains that you want to use as the parent domains for your hosts and you already have all required ingredients ( a linux server with a static ip that's always up and runs bind and apache with php )
Q: How do you run your own dynamic DNS service?
A: get DDNS Server, read the README file and set it up. The nice thing about this script is that it uses the same protocol as the popular DynDNS service so any dynamic ip update client like ddclient would work with this script. Of course the client would have to allow you to specify the hostname or ip and port of the dynamic dns server.
Oh and one more thing, if you're running bind you're probably running it in a chroot environment so you'll have to apply the following patch to DDNS Server ( current version 1.0.0 ) to make it work with the chrooted bind :
This patch adds a new configuration option named BIND_CHROOT ( in config.php ) that has to be set to the path of the chroot environment where bind runs. Eg. on centos 5.3 this is /var/named/run-root .
For any questions or suggestions hit the comments.