Currently the documentation says:
A database that is marked datallowconn = false in pg_database is
assumed to be properly frozen; the automatic warnings and wraparound
protection shutdown do not take such databases into account. Therefore
it's up to you to ensure you've correctly frozen a database before you
mark it with datallowconn = false.
http://www.postgresql.org/docs/8.1/interactive/maintenance.html#VACUUM-FOR-WRAPAROUND
When a VACUUM FREEZE is done, does "datvacuumxid = datfrozenxid"? If
so, should VACUUM skip warning about any databases that are
"datvacuumxid = datfrozenxid and datallowconn = false"?