Apple has released Mac OS X 10.5.4 via the Mac OS X Software Update:
This represents a rapid turnaround for Mac OS X version updates, with the last point upgrade having been released just one month ago (May 28th). The new update, however, is a requirement for some of the features of Apple’s upcoming MobileMe service.
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.