WordPress 3.1 released, features massive writing workflow and admin overhauls
The most notable new new change is a massively overhauled writing interface, which should make the entire publishing process both more streamlined and less intimidating for new users. Linking back to old posts and pages is now a lot easier, too (just click a button and start searching), and lots of rarely-used blocks have now been hidden by default (they can be re-enabled in Screen Options).
There's a new Admin Bar, which appears at the top of every page and makes moving around your blog's back end a whole lot easier. For power users and theme makers, there are now Post Formats, which significantly increase the flexibility of WordPress layouts.
For a complete list of the changes and new features, check the WordPress 3.1 Codex page.