One year after the release of Debian GNU/Linux 4.0, codenamed ‘etch’, and nearly three years after the release of Debian GNU/Linux 3.1, security support for Debian GNU/Linux 3.1 will cease at the end of March.

The Debian project released Debian GNU/Linux 4.0 on 8 April this year. In a statement today the development team said: “Users and distributors have been given a one-year timeframe to upgrade their old installations to the current stable release. Hence, the security support for the old release of 3.1 is going to end in March 2007 as previously announced.”

Bit more here or Debian's Offical Statement