I'm assuming it was the 10.5.6 update, possibly a security update, I don't know. But Postfix broke again after the install finished. Same nonsense with many log entries about files not being owned by Postfix, as before, running a new 'make upgrade' fixed that up.
Had one last problem that was new, Postfix launched, but wasn't receiving mail. main.cf had an entry for inet_interfaces that had previously been set to 'all', but was no longer. Changing that back and relaunching Postfix cured the issue.