Red Hat Enterprise Linux 3 will be end-of-life on 31 October 2010 (about six months away). Red Hat gives details about their product lifecycle on their web site.
RHEL 4 will reach end-of-life on 29 February 2012, and RHEL 5 will reach end-of-life on 31 March 2014 – so for them, there is several years left. The CentOS distributionCentOS distribution has CentOS 4 and 5 available for download – CentOS is an open source build of Red Hat Enterprise Linux.
RHEL 6 entered beta a little while ago, and is available for testing. CentOS 6 does not seem to exist yet.