Periodically someone hacks together something that works, last big effort I'm aware of was in 2006, and then it bit rots away. I'm sure we'd get some user uptake on the result--MySQL corruption is one of the top ten cause of a MythTV system crashing. The developers are so resistant to database-neutral design that you'd need quite the thick skin to try and get something into their mainline though, which means someone who tried adding PostgreSQL support would likely have to run a parallel branch for some time, expecting regular breakage. The only thing on their radar as far as I know is SQLite.
There was a good overview circa 2004 at http://david.hardeman.nu/files/patches/mythtv/mythletter.txt , haven't done a deep dive into the code recently enough to comment on exactly what has changed since then. That gives a flavor for the fundamentals of the design issues though.
-- Greg Smith 2ndQuadrant US Baltimore, MD PostgreSQL Training, Services and Support greg@2ndQuadrant.com www.2ndQuadrant.us