Developers who need to use System.Web.HttpUtility in their client apps and had to reference System.Web.dll and therefore target NET4
full (System.Web.dll is in Full) , can now target the NET4 Client
Profile by using the new System.Net.WebUtility class which is in
System.dll (System.dll is in NET4 Client Profile).
System.Net.WebUtility includes HtmlEncode and HtmlDecode. Url encoding
can be accomplished using the System.Uri class (also in System.dll).
From http://blogs.msdn.com/b/jgoldb/archive/2010/04/12/what-s-new-in-net-framework-4-client-profile-rtm.aspx