The latest Community Technology Preview (CTP) for SQL Server 2008 (formerly known as "katmai") is available for download via the Microsoft Connect website. As is mentioned on the right hand side of the download page you need to register for the CTP BEFORE downloading the code otherwise you'll be disapointed!
There's a wealth of information about SQL Server 2008 via the Microsoft SQL Server home page including the following:
- SQL Server 2008 Is Coming! Learn More About the New Capabilities in This Upcoming Release - this is the main site - it contains links to videos and plenty of technical information such as