Security Update 2008-003 / Mac OS X 10.5.3 is an important one for a few reasons, but relevant here are some exploits that have existed for a bit which Apple patched:
Apple Pixlet Video
Flash Player Plug-in
CVE-ID: CVE-2007-5275, CVE-2007- 6243, CVE-2007- 6637, CVE-2007-6019, CVE-2007-0071, CVE-2008-1655, CVE-2008-1654
It's always good to wait a week or so for the dust to settle whenever you update a production Mac (ie..a Mac that you use to make money with or use for an important project) but afterwards if the discussion boards are clear-ish go ahead and update.