The WordPress team released WordPress 2.6.5 earlier today. This release addresses a potential XSS (cross-site scripting) attack under some server configurations, plus adds some bugfixes for some other minor issues. As noted in the official announcement, there was no official 2.6.4 release. There was an attempt to fool people into downloading a fake release under that number, so it has been skipped in the official release numbering, to avoid confusion.
I would like to take this opportunity to point out the WordPress project entry on Freshmeat. Freshmeat is a site which tracks the latest versions of various open source projects. By registering on the site, you can subscribe to your favorite projects, and receive updates by email whenever a new release is announced. While you’re there, don’t forget to rate your favorite projects (via either the rating dropdown on the project page, or the project toolbar icon that looks like a speedometer on the main page project listing).