Joomla! 1.5.6 released
It is a security patch, to fix an important bug.
SECURITY [HIGH] Fixed security hole in reset logic to check for proper token length.
To upgrade from 1.5.5, you just need to replace following files.
/components/com_user/models/reset.php
/changelog.php
/includes/framework.php
/administrator/includes/framework.php
/libraries/joomla/version.php
/libraries/joomla/environment/uri.php