[Warning: Do not test on your live website blog because the script is changing and you could lose all your data. Test on your localhost.]
An early beta tester version of WordPress 3.0 Beta 1 is available for download now, it’s a merge between WordPress and WordPress MU (WordPress Multi User) WordPress that allows multiple blogs to operate from a single installation. In this version, it allows user to create network of sites by adding a few lines in your wpconfig.php.
What’s New in WordPress 3.1:
1. Merge of WordPress MU + WordPress.org: So now you can have multiple sites in one installation. 
2. Custom Post Types/Better CMS: This was originally a Feature planned for wordpress 2.9, but due to some technical difficulties, this feature did not make it in. but now WordPress 3.0 will be Combining Post Types and Taxonomies will make WordPress a much more robust CMS option. This feature will allow you to have different type of posts for example Product listing, portflios and then Normal blog posts, which is a Great feature indeed.
3. Better Navigation Menus: Custom Woo Navigation will be included to the core in this release in order to allow for better menu management.
4. New Default Theme: WordPress 3.0 comes with a new default theme known as 2010. From this year on, the goal is to have a new theme for every year.
5. More customization: Custom Background Support: Support for custom background can be called by adding a line in your functions.php.
6. Author Specific Templates: In WordPress 3.0, you can create specific templates for each author. The function get_author_template(); has been expanded in wp-includes/theme.php.
7. Canonical Plugins: Canonical plugins are developed by a community instead of one developer, so if one person steps down, the plugin does not die.
Now you can easily organize your menu
Things to test
Play with new default theme, Twenty Ten, including custom background and header options; Custom Post Type functionality has been beefed up. It’s really easy to add new types, so do that and see how it looks!; WordPress MU users should test the multiple sites functionality to make sure nothing broke during the merge. Install WordPress Beta Tester plugin, switch over to beta and download WordPress 3.0 Beta 1!,” states WordPress dev blog.
Already have a test install that you want to switch over to the beta? Try the beta tester plugin. Testers, are encourage to use the wp-testers mailing list to discuss bugs you encounter. WordPress 3.0 Beta 1
WordPress 3 Feature