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:
Raj has posted an interesting post about SQL 2008 and it's features.
I'd like to explore the new version