An official announcement from wordpress.org:
WordPress 3.0, the thirteenth major release of WordPress and the culmination of half a year of work by 218 contributors, is now available for download (or upgrade within your dashboard). Major new features in this release include a sexy new default theme called Twenty Ten. Theme developers have new APIs that allow them to easily implement custom backgrounds, headers, shortlinks, menus (no more file editing), post types, and taxonomies. (Twenty Ten theme shows all of that off.) Developers and network admins will appreciate the long-awaited merge of MU and WordPress, creating the new multi-site functionality which makes it possible to run one blog or ten million from the same installation. As a user, you will love the new lighter interface, the contextual help on every screen, the 1,217 bug fixes and feature enhancements, bulk updates so you can upgrade 15 plugins at once with a single click, and more.

Version 3.0 of WordPress is in the beta release stage. It is scheduled for final release in May 2010. Below is the list of some of the enhancements and fixes which WordPress 3 will bring.

• New menu management feature
• Improved custom post types
• Improved custom taxonomies including hierarchical (category-style) support
• New theme “Twenty Ten” is the only theme in the WordPress distribution, and is now designated the ‘fallback’ theme
• Improved child theme support; child theme use is highly encouraged and as described in the Child Themes article, very simple to accomplish
• The Default (Kubrick), and Classic themes, are no longer included in the WordPress distribution, but are available in the Theme repository
• Bulk theme update ability
• Standalone WordPress and WPMU code merged
• Configure a Network (multisite/WPMU)
• Allow Tab indenting, Shift-Tab (outdenting), and multi-line tabbing, in plugin/theme editors
• Improved revision comparison user interface
• Allow for removing all tags in quick edit

You can read more at wordpress.org/…