Summary: Longhorn promises to be a great platform for least privileged applications. Get started today by writing managed code, first of all. When building desktop applications, make them LUA-compliant (and use the Windows Application Verifier to help check your work)
http://msdn.microsoft.com/longhorn/default.aspx?pull=/library/en-us/dnlong/html/leastprivlh.asp
Source: Jerry’s Security Weblog