This Release Candidate 4 build includes the security fixes and enhancements included in 3.0.5 and addresses about two dozen additional bugs.
This includes fixes for:
Deleting a user and reassigning their posts to another user.
Marking multiple users or sites as spam in multisite.
PHP4 compatibility.
To test WordPress 3.1, you can use the WordPress Beta Tester plugin
More info and download: http://wordpress.org/news/2011/02/wordpress-3-0-5/