Apple's patch notes:
This update is recommended for all Safari users and includes improvements to performance, stability, and security including:
• Improved Full History Search performance for users with a large number of history items
• Stability improvements for 3rd-party plug-ins, the search field and Yahoo! Mail
Apple also points users to its Security site for details on the security contents of this update, but as of this writing it has not yet been added.
You can download the update through Software Update in Mac OS X, through the Apple Update utility in Windows, or directly from the Safari download Web page. In Snow Leopard for Intel, it's a 29.8MB download through Software Update.