I tried to install the AdventureWorks Sample database on a named instance of SQL2008 and receive the following errors in the Application error log.
 
Event Type: Error
Event Source: MSSQLSERVER
Event Category: (2)
Event ID: 948
Date: 11/26/2007
Time: 5:38:51 PM
Description:
The database 'AdventureWorks' cannot be opened because it is version 639. This server supports version 612 and earlier. A downgrade path is not supported.
 
 
Event Type: Error
Event Source: MsiInstaller
Event Category: None
Event ID: 11722
Date: 11/26/2007
Time: 5:38:55 PM
Description:
Product: Microsoft SQL Server 2008 AdventureWorksDB -- Error 1722.There is a problem with this Windows Installer package. A program run as part of the setup did not finish as expected. Contact your support personnel or package vendor. Action InstallDatabase, location: C:\Program Files\, command: "C:\Program Files\Microsoft SQL Server\100\Tools\Binn\SQLCMD" -S (local) -E -d master -Q "CREATE DATABASE AdventureWorks ON (name = 'AdventureWorks_Data', filename = 'C:\Program Files\Microsoft SQL Server\100\Tools\Samples\AdventureWorks_Data.mdf'), (name = 'AdventureWorks_Log', filename = 'C:\Program Files\Microsoft SQL Server\100\Tools\Samples\AdventureWorks_Log.ldf') FOR ATTACH;"
 
It looks like the installer is trying to attach the sample database to the default instance instead of the named instance. During the installation process I selected the named instance - see the attached sql_install_error.jpg.