The official release reads:
The Joomla Project is pleased to announce the immediate availability of Joomla 1.5.7 [Wovusani]. This is a security release and contains a number of bug fixes, improvements as well as security fixes. It is strongly recommended that users immediately upgrade. It has been nearly four weeks since Joomla 1.5.6 was released on August 12, 2008. The Development Working Group’s goal is to continue to provide regular, frequent updates to the Joomla community.
Joomla.org is really taking security seriously. The new Joomla Security Strike Team is now firmly established and there is a very useful RSS feed that will notify webmasters of security issues. Subscribe to Joomla! Security Announcements