After my recent hardware woes, I've had a little more success getting MythTV back up and running. I bought another cheap Dell workstation to act as my new Myth server. After a little fiddling, it's up and running just fine, with three tuners and so far 600 gigs of SATA drives (to be upgraded to 1.6TB tonight when I buy another SATA interface card). It's also hosting SqueezeCenter which drives all my Squeezeboxen for music around the house.
Previously I had a fanless, diskless front-end in the lounge room, using a Via EPIA board with video decompression done on its video chips. This worked fine and was beautifully silent, but doesn't support high definition, and I now have a high definition telly. The telly came with a "free" (really $300) Playstation 3, but the MythTV client on PS3 is being held up by Sony blocking hardware access to the video decoding hardware in third-party operating systems. That's a real shame.
What I've discovered is that the PS3 is a very powerful UPnP client, and MythTV makes its recordings available over that protocol. So the PS3 is able to play back 1080i recordings taken over the air without any problem.
Downsides of doing it this way are that you lose some functionality. The UI is just a list of recordings, without the cool context you get in the Mythfrontend. You can't delete recordings, you can't schedule (though I suppose the PS3 web browser pointed at Mythweb will do that) and features like commercial skipping aren't supported. UPnP effectively treats the MythTV server as a file share of video files. Hopefully at some point in the future the PS3 will natively run a Myth frontend, but for now this works pretty well.
I'm also sharing out my music and other video files from the server using the MediaTomb UPnP server. It's quite flexible, and explicitely supports PS3. A nice feature of it is you can transcode anything on-the-fly for cases where a media type isn't supported. For example, PS3 doesn't support OGG, FLAC or Matroska. So you can set up a rule in MediaTomb to transcode these into something the PS3 does support. Quite neat!