currently the list is very simple:
and hopefully, the new HD-PVR when we get drivers, it is the first on the list for a new device template...
Of course, these are the best tuners to use because they are supported directly in both LinuxMCE's direct AV functionality, and in MythTV.
To support more, you need a TV tuner that can output an MPEG-2 (or maybe with a bit more work to Xine, an MPEG-4 stream)....
You CAN use MythTV supported tuners, but you will have to configure the tuner card manually in MythTV setup, and you will not have access to the Direct AV functionality.