In a nutshell, .NET Compact Framework 3.5 supported features for WCF include: messaging-layer communication, WS-security, and http and mail transports.  All of these a subsets of what the desktop supports, with the exception of the mail transport, which is new with NetCF. There is also some plans to provide a tool to generate code for a NetCF app that will allow the app to call a service on a desktop.