Upgrading to Ubuntu 8.04 has been fairly successful, but it took me a while to find out what had stopped my TV from working. Nothing seemed to be different, same xorg.conf file, no errors in /var/log/Xorg.0.log, "space" in the desktop for the TV, but the TV was black.
I finally found this nvnews forum post that gave the clue, and adding the following to the "Device" section in my xorg.conf file did the trick:
Option "TVOutFormat" "SVIDEO" Option "TVStandard" "PAL-G"
