Most people do not run SQL on their Windows client PCs, with the exception of MSDE.  Windows Vista will not support MSDE (aka SQL 2000), so applications dependant on SQL will need to be moved to SQL Express, which in simple terms is SQL 2005 MSDE on steroids.  I have checked and Microsoft will be releasing updates to it's packages that are supported on Vista to move them to SQL Express.  Obviously, if you are looking at Vista, check with other 3rd parties to ensure they are doing the same. 

More details can be found at Running SQL Server on "Microsoft Windows Server Longhorn" or Microsoft Windows Vista.

ttfn

David