On Saturday, 28 March 2009, the Joomla Development Team has released a new version of Joomla! 1.5, which is now the 10th maintenance release: 1.5.10. "Wohmamni" - that's the name of this release is a security release and users are strongly encouraged to upgrade immediately. Although no critical errors have been found, you are at risk for XSS-attacks and CSRF (cross-site request forgery) if you don't update your installation.
Make sure you read the full announcement at joomla.org.