WordPress has just released WordPress 3.1.2 Security Update which fixes a loop hole allowing Contributors of multi-authored WordPress blogs to improperly publish posts. Since 3.1.1, Archive pages had 404 issues for pages like /tag/tag1+tag2/, this release also patches that problem.
If you run a multi-authored blog which allows users to register as contributors or if you have untrusted contributors, considering upgrading WordPress as a soon as possible. Although this affects only WordPress 3.1+ version, it’s still wise to upgrade considering all the new features available for with WordPress 3.1
Backup your WordPress database before upgrading. You can use one-click update from Your Dashboard or manually download it from WordPress.org