Ubuntu Linux 8.10 release candidate beta version has been released and available for download from mirrors. The final stable version will be released on October 30th, 2008. But, if you would like to test latest version try out Intrepid Ibex RC beta version.
Upgrading from Ubuntu 8.04 To 8.10 beta
To upgrade from Ubuntu 8.04, press Alt+F2 and type in "update-manager -d" (without the quotes) into the command box. Update Manager should open up and tell you: New distribution release '8.10' is available.
Click Upgrade and follow the on-screen instructions.