The latest from the Team WIT Tools blog is an excellent post about the compatibility between Team Explorer versions and Team Foundation Server versions. (Link) The highlight is and easy-to-grok matrix of Clients to Servers.
“TFS compatibility is taken very seriously - both new clients accessing older server versions and older clients accessing newer server versions. As such, we have worked to make the new TFS 2010 clients work with the TFS 2008 servers (in addition to TFS 2010 servers). We are also ensuring that TFS 2005, TFS 2008 and MSSCCI clients will work with new TFS 2010 servers, although patches are required to ensure this. Many of the features in TFS 2010 have components on both the client and server that work together to provide the full benefit. Using either an older server or older client means that some of the TFS 2010 features will not be available to you, but we have taken great strides to ensure that key features (old or new) work. In addition, API compatibility support was maintained in the object model and web services, so programs written against the old client or server versions should continue to work as designed . . .”