I wanted to experiment for a while with .Net Framework Client Profile. So I created windows forms application to test which namespaces are included in this "client" version of the framework. I added reference all System.* assemblies. And what is the result:
If you check "client-only framework subset" option all the namespaces not included in the client subset will appear with exclamation sign. If you have source using some of these namespaces it will of course fail to build.
Read more about .NET Framework Client Profile :
Walkthrough: Deploying a .NET Framework Client Profile Application by using Windows Installer
Walkthrough: Deploying a .NET Framework Client Profile Application by using ClickOnce