WordPress 3.1.1 is a recommended security update, also improves performance
WordPress 3.1.1 contains many security fixes, and as such not updating as soon as possible may leave your blog vulnerable to attacks. Among the fixed issues are two relating to the media uploader, an XSS flaw, and one that caused a PHP crash in certain conditions when handling "devilishly devised links in comments."
Alongside the security patches come performance improvements, fixes for taxonomy and some permalinks, as well as various things that have been causing plugin compatibility issues.
If you're currently a WordPress.org user, you should have already gotten notified about the update in your Dashboard. If not, or if you aren't a user just yet, you can download WordPress 3.1.1 from the official website.