The second beta of WordPress 3.1 is now available to download. It is not recommended to run a normal live site.
WordPress say “If all goes well, we hope to release WordPress 3.1 to the world at large by the end of the year, though that is (as always) subject to change/dependent on how the beta period goes. The more help we get with testing and fixing bugs, the sooner we will be able to release the final version.”
Download and more info: http://wordpress.org/news/2010/11/wordpress-3-1-beta-1/