As announced on the official Joomla Project Developers Blog a few weeks ago, Joomla 1.0 is headed into the sunset next July.
Longtime Joomla volunteer Wilco Jansen writes:
But now it's time to say our farewells to our old friend Joomla 1.0. As of July 22, 2009, the Joomla 1.0.x series will no longer be supported.
As a user, is it required that you upgrade from Joomla 1.0.x? Absolutely not since security upgrades will be supported until this date.
But if you're a user who hasn't yet upgraded to Joomla 1.5, you should do so in order to start reaping the benefits the latest version has to offer. If you're a developer in that small minority who hasn't yet become Joomla 1.5 native, this is your last call to join the majority of developers who have already discovered the power and ease of the Joomla 1.5 series.