RHEL backported one additional year
At the 2008 Red Hat summit in Boston, Red Hat outlined to support Red Hat Enterprise Linux for new hardware and installation media one year longer than it did in the past.
This is a major event. In the past Red Hat offered new hardware support, bugfixes and feature enhancements (dubbed full support) for 3 years after the initial release. But now that will be for 4 years after initial release. New installation media will be release up to 5 years after initial release !
That means that RHEL5 (and CentOS 5), released on March 2007, will have full support until March 2011, new media releases until March 2012 and will have security updates until March 2014.
If you were running RHEL4 (or CentOS 4), released on Feb 2005, than the upcoming 4.7 release will not be the last, but there will be at least a 4.8 and 4.9 release until May 2009, new media releases until May 2010 and from then on the normal security updates will keep coming in until Feb 2012.
This matches much better with the normal hardware lifecycle within companies. Usually hardware is decommissioned after 4 to 5 years and since a release is often not being deployed in production when it ships, but more or less a year after initial release. That means there is a much better (and wider) overlap between supported releases.
This is a major improvement to Enterprise Linux as a whole and I am very keen to see how Novell and Canonical are going to react to Red Hat's newest twist.