have you ensured that xv overlay is properly configured in your xorg.conf? per old notes i have from
when i was running an ati card, i had to add the following to the "Device" section of xorg.conf for
the video card. as i recall, ati-config was pretty dodgy generating a correct xorg.conf (not that
nvidia is much better in that respect).
as a troubleshooting measure, you can also try running mythfrontend from a terminal window- then
Option "VideoOverlay" "on"
Option "OpenGLOverlay" "off"
launch livetv. when the video starts, alt-tab back to your terminal window and see if it is throwing
xv-related errors or any other errors that might help track this down.