Apple made available the maintenance release of Mac OS X version 10.6.7 via their web site and Software Updates. You can either use the Software Updates menu item (under the Apple menu) to upgrade your Mac or you can download the Combo Updater from Apple's Support area.
As promised Apple just made available Mac OS 10.6.4 for download.
There are two Knowledge Base articles to describe what have changed in this latest OS X release. The first is "About the Mac OS X v10.6.4 Update" and the other is an "About the security content of Security Update 2010-004 / Mac OS X v10.6.4".
As with all other OS X updates I recommend that you download the Combo Update and then apply it to your Mac.