Preview at WWDC likely in June, followed by September launch.
Apple Releases Mac OS X 10.5.4 [Updated: Safari 3.1.2 for Tiger]
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.
Update: Alongside OS X 10.5.4, Apple also released Security Update 2008-004 for users running OS X 10.4.11 and 10.5 - 10.5.3. This security update appears to be included in the OS X 10.5.4 update but has also been made available as a standalone update.
Finally, Apple released Safari 3.1.2 for Mac OS X 10.4.11. This update addresses a WebKit vulnerability that was fixed for Windows XP and Vista users in Safari 3.1.2 for Windows (released June 19th) and for Leopard users with the 10.5.4 update.