I wanted to continue to use Ubuntu Server in my VMs. I like their approach to long-term support, the server install, and the way they provide a pre-configured LAMP server.
As I read about how to build one, I stumbled across a post from someone else who was doing what I wanted to do, and had problems. That scared me off just blindly stumbling ahead, so now I have more research to do.
I'm pretty sure it should be relatively easy to have a kernel that works on top of Xen, and on which I can run an otherwise standard Ubuntu 6.06 install, but it's going to take some digging... This is harder than I wanted it to be.