Looking at the logs in the Console application, there seem to be a lot of errors of the form:
tdb(unnamed): tdb_open_ex: spinlocks no longer supported
If I get rid of
/private/var/db/samba/secrets.tdb, then there are more errors
complaining about different
.tdb files in the
directory. So I tried getting rid of the whole
/private/var/samba/ directory (after having
saved off a copy, of course), and I can suddenly see the Mac from the Windows machine again.
Of course, this BFMI approach isn't really a solution to be recommended: I've got no idea what the files were there for and whether this has just made my Samba connections dreadfully insecure.