In an effort to provide customers with more secure products, Microsoft Windows Server "Longhorn" and Microsoft Windows Vista will only support SQL Server 2005 Service Pack 2 (SP2) or later when it becomes available. Earlier versions of SQL Server, including SQL Server 2000 (all editions including Desktop Engine edition, a.k.a MSDE), SQL Server 7.0, and SQL Server 6.5, will not be supported on Windows Server "Longhorn" or Windows Vista. Customers running applications with these earlier versions of SQL Server should consider evaluating and upgrading to SQL Server 2005, which was designed to take advantage of the upcoming security and performance enhancements in the operating environment.
That means, applicaiton developers are to be aware that running an older version of SQL as stated above, the application has higher chances of "incompability" on the soon to be RTM'ed Vista and Longhorn thereafter. Setup Engineers and Application Packagers must inform or follow-up with the application programmers/developers OR vendors to have this information relayed for application stability in Vista & Longhorn.
Microsoft has outlined some necessary information how to upgrade to current supported version along the earlier message release. Read more here.