WordPress 3.1 has now been released. Like all releases it is usually a good idea to wait a couple of weeks before you update to allow for themes and plugins that need to update. Before you do update, make sure you back up your site and database as well (just in case something goes wrong!). A good free WordPress plugin is Back Up and Move.
Here’s a quick video tour of the main changes in WordPress 3.1.
Some of the other features (which are ones that will excite developers) are listed at the WordPress News page.