This seems to be a common problem with MythTV, OpenOffice Impress, and other applications that have fullscreen modes. The fix is simple. Install the compizconfig-settings-manager package onto your system. On Ubuntu, the command to install it is:
After the installation finishes, you will have a new application, CompizConfig Settings Manager, listed under: Gnome System menu > Preferences.
Open it and start typing 'workarounds' in the Filter search bar. Select the Workarounds plugin and enable the Legacy Fullscreen Support option as shown in the screenshot below.
Now, the next time you open MythTV or another fullscreen application you won't get the Gnome panels showing on top of everything else.