If you have a Hauppauge PVR-500 card and recently upgraded Ubuntu to latest 10.04 Lucid, then you might have had problems viewing live TV.
The quick fix might be to just install the package “ivtv-utils” and to reconfigure the video input/sources with mythtv-setup.
I also had a problem with disconnects and warnings of wrong protocol version. I cleaned out the database and deployed a new database with the sql file from /usr/share/mythtv/sql. That might be because of some badly configured IP-addresse for the backend and master, but I could actually not see why it changing from my LAN IP-address to 127.0.0.1 would have anything to do with it.
Anyway… Try installing ivtv-utils first. After installing that package the MPEG2 capture card selection becomes available. First I tried using the v4l option, but that didn’t work, not even any useful log messages to work with.