On Wednesday afternoon, Apple posted Safari 3.1.1 for Windows and Mac OS X. "This update is recommended for all Safari users and includes improvements to stability, compatibility and security," Apple said.
The security fixes are explained further at Appleis Security Update Page.
Two of the four security fixes are for the Windows XP and Vista platform. The other two related to Webkit apply to both Windows and Mac OS X, starting with Mac OS X 10.4.11.
The update appears in System Preferences... -> Software Update. A stand alone version is available on Appleis Downloads page. It is 39 MB.