As of todays release announcement, PostgreSQL 8.3 is no longer a supported version. PostgreSQL 8.3 is end of life in according with the 5-year support policy for all PostgreSQL major versions.
If you are running on any of the 8.3 versions, you should upgrade your system to 8.4 or newer immediately. If this cannot be done immediately, you should at least upgrade to 8.3.23 in the meantime. When upgrading, it's likely worth upgrading to 9.2 or at least 9.1, and not just to 8.4 which only has a bit over a year before it also goes end of life.
Any users of PostgreSQL 9.2, 9.1, 9.0 or 8.4 should still look at upgrading their systems to the latest minor release as of todays updates, since they contain both security and stability fixes. Minor version upgrades are, as always, just a matter of replacing your binaries and restarting the database. Automatic updates should also be out on the yum and apt repositories shortly.