For some strange reason the /var/run/mysqld directory wound up with incorrect permissions to allow the user mysql to write to it. Neither the mysqld or mythbackend init.d scripts caught the failure, so the machine was sitting with neither service running for the better part of a day (since I rebooted fairly early this morning to try to get ALSA working).

Fix was simply to fix the ownership of the mysqld directory, but wish the init.d scripts would have noticed the failure and told me the services were down, rather than silently ignoring it.