Unfortunately its not as simple as you would think, SQL Server 2008 install requires the .NET 3.5 framework to be installed or will install it as part of the pre-req checking. Changes in Windows 7 can prevent SQL Server 2008 from installing the .NET 3.5 framework pre-requisite. Instead you may need to install the framework separately using the “Run this program in compatibility mode”. Read here for more information.
UPDATE 2009-06-29: It seems there are similar issues with SQL2005 as well, have a look SQL Server version has a known compatibility issues with Windows 7 RC. This warning is genuine and applies to all SQL Server editions, including all Express editions.
Gareth