I know what you're thinking...  Isn't UAC just a Windows Vista thing?  Nope.  Get all the details at Chris Henley's blog post on the subject. Chris is on a rampage and is cranking out a bunch of Windows Server 2008 screencasts.  See this one at :

http://blogs.technet.com/chenley/archive/2007/08/30/windows-server-2008-user-account-controls-video.aspx