I know this is not an answer, but there is a knoppix remaster called KnoppMyth just to make easy the MythTv installation.
Once you boot the CD you can select to install the whole distro with MythTV on your HD (this is necessary if you want to run the MythTV server) or to run the mythTV client right fron the CD.

You can find it at http://www.mysettopbox.tv/knoppmyth.html

ciao
Stefano