Apple has just released a new security update for client and server users of Mac OS X 10.4 (Tiger) and 10.5 (Leopard). This Security update 2008-005 is "recommended for all users and improves the security of Mac OS X," and includes fixes for a dozen or so features in Mac OS X.
This update fixes a QuickLook bug where loading a malicious Microsoft Office file could lead to "arbitrary code execution." Doesn't sound too good to me! It also fixes a DNS vulnerability that has been highly discussed over the past week (and which many other vendors have already patched).
You can download this update by opening Software Update (Apple menu > Software Update). You can also find more information on what this update fixes by looking at Apple's support note.