Debian provides security updates for old stable releases for one year after a new distribution has been released. Yesterday, Debian announced that the support for Debian 3.1 (sarge) is coming to an end this month, as etch was released last year in April, sarge will be supported until March 2008.
Anyone still running Debian sarge, should really be thinking seriously to finally update to etch asap. ;-)
Debian’s Official Announcement: http://www.debian.org/News/2008/20080229