Some of the namespaces in the Team Foundation client object model aren't in namespace.dll. Sometimes, the namespace is spread over more than one assembly. Here's a table to help you find the namespace your interested in, and to make sure you have the whole thing. (These are installed with Visual Studio 2010 at Program Files\Microsoft Visual Studio 10.0\Common7\IDE\ReferenceAssemblies\v2.0).

Namespaces and Assemblies