I got a netboot system working yesterday. able to boot appliances into a Centos 5.2 installation. I'm now working on migrating our old kickstart stuff into this new system and try it out.
In the process I've discovered that we're using CentOS 4.4 as the base for this appliance build. Centos 4.4 was a little tricky to find, everyone redirects you to the latest 4.X which is 4.7, and most mirrors don't provide anything below 4.6. I found them at the Vault just in case you need them too.