Jaime Rodriguez in the team put together a detailed view of the Client Profile a while back, which is a subset of assemblies already contained within .NET Framework 3.5 Service Pack 1. With all the noise about Silverlight, this hidden gem seemed to get lost in the mix. Its worth looking at this again. The Client Profile subset is focused on fulfilling the needs of Client applications and comes with a number of capabilities :-
To get a lot more information about this, visit this post (http://blogs.msdn.com/jaimer/archive/2008/08/20/client-profile-explained.aspx)