This patch seems to fix the case for me:
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=94bdb198813b079467d7ed07c6f72ac896da7161
just run: CREATE EXTENSION citext FROM unpackaged; and the table and indexes can be used as with 9.0.5.
Thanks for your quick help.
Regards,
-Rudolf.