From the Ubuntu weekly newletter #112:
This means no more updates and security patches. Also soon the repositories will be closed, you will unable to install anything from them.Ubuntu 7.04 Updates
Ubuntu 7.04 will reach the end of its support period on Sunday, October 19, 2008. After that date, no further listing for it will be posted here. You are encouraged to upgrade to Ubuntu 7.10.
Your choices are
1) Upgrade or clean install to Gutsy Gibbon. Its end-of-life is in 6 months - April 2009.
2) Clean install Hardy Heron - Long Term Support. Its end-of-life is Desktop: April 2011 and Servers: April 2013.
3) Clean install Intrepid Ibex - will be stable on 30 October 2008. Its end-of-life is April 2010.