Microsoft has releases a Community Technology Preview (CTP) in June for the upcoming database product SQL Server 2008 aka Katmai. You can download this first CTP of Katmai from Microsoft Connect, http://connect.microsoft.com
Unfortunately, in this first CTP release of SQL 2008 code name Katmai sample databases are not installed by default. But you can download the installation packages of the sample databases and install by yourself.
I could finally point out where the msi packages are reside in the CodePlex. Go directly to the following link http://www.codeplex.com/MSFTDBProdSamples/Release/ProjectReleases.aspx?ReleaseId=4605 for Adventure Works databases for various systems.
AdventureWorksLT.msi is small in size and after you complete the download from the CodePlex SQL Server Sample Databases pages, you can run the installer package and complete the installation of the AdventureWorksLT database in minutes.