In April, 2011, a serious pg_upgrade bug was fixed:
http://wiki.postgresql.org/wiki/20110408pg_upgrade_fix
Unfortunately, it has been discovered that the fix did not correct
upgrades *from* Postgres 8.3. A new fix has been committed for future
releases of Postgres branches 8.4, 9.0, and 9.1. The details are here:
http://wiki.postgresql.org/wiki/20110905pg_upgrade_fix
Again, this only affects users who use pg_upgrade *from* Postgres 8.3.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us