I wasn’t sure this was actually a new update until I checked both Macs in my home as there was so little information to go off of. Basically this is a typical security update available for those who installed Safari Beta 4. Apple says nothing else about the update and it is available via software update.
This update is recommended for all users of the Safari 4 Public Beta and includes the latest security updates.
For detailed information on the security content of this update, please visit this site: http://support.apple.com/kb/HT1222.