As predicted, I did something horribly wrong to the rig. I couldn't get 10.4 to run Xen so I switched back to CentOS. CentOS runs Xen nicely, but it didn't like my display and I couldn't get MythTV to function well without it. The NVidia drivers that I could install just wouldn't work well.
For generic host configurations, CentOS works nicely. My problem is that I only have the one system and I'm unwilling to give up specific features/services.
In any case, I've moved back to 10.4 and am currently playing with KVM. I have a number of dedicated guest machines running and am currently learning how to grow qcow2 images.
This second part has been a bit difficult in that qemu has some quirks when it runs live CDs (such as qparted which is needed to resize a partition inside of an imaged filesystem). An example of this is where, while running the qparted live CD under qemu, the mouse "sticks" to the right-hand side. It took a little searching but this issue was overcome by running "export SDL_VIDEO_X11_DGAMOUSE=0" on the command line just before starting qemu. (Hint: once you're done with qparted, you'll want to close the terminal window to destroy that export.)
In any case, KVM seems to be meeting most of my needs. I'll start adding notes to the wiki shortly, in with what's already there for ESXi, Virtualbox, and Xen (ouch).