Apple has posted Security Update 2008-002 for PowerPC and Intel Macs on Tuesday. The Update generally applies to Mac OS X 10.4.11 and 10.5.2.
"Security Update 2008-002 is recommended for all users and improves the security of Mac OS X. Previous security updates have been incorporated into this security update," according to the update now available in System Preferences -> Software Update.
A standalone installer, as always, can be downloaded from the Apple downloads page.
The list of all Apple security updates after 25 January 2005 with more detailed information is in Apple KB# 61798.