Hi, everybody!
I already solved it. Thank you.
First of, I forgot to mention the version(), so FYI, 7.4.1.
I followed Tom's advice in
http://archives.postgresql.org/pgsql-general/2004-03/msg01073.php
I shut down the PG instance.
postgres -D $PG_DATA -O -P $DBNAME
=# REINDEX DATABASE tbc_gamma
<EOF>
-P was necessary for the standalone backend to ignore the (damaged) indexes.
Done.
Hope this helps someone else someday.