The 10.5.4 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

For detailed information on this update, please visit this website: http://support.apple.com/kb/HT1994.
For detailed information on security updates, please visit this website: http://support.apple.com/kb/HT1222.

Apple began seeding Mac OS X 10.5.4 to developers only a month ago, it was rumoured that the update would come quickly as it packs updates required for MobileMe.