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