The short version is that TFS 2010 will support most of the clients used with TFS 2008 today.
There will be places where VS 2005/2008/2010 behave differently but for me it makes sense that 2005 and 2008 will work with their current feature set and only the latest features will be available in 2010.
One small noticeable difference in 2005 and 2008 is that the URL to TFS changes so that we can specify which project collection in TFS to connect to. The default collection (where all upgraded 2008 projects will be placed by default) would require the following setting in Team Explorer 2008:
For older clients to operate correctly with the new TFS version a “Forward Compatibility Update (GDR)” needs to be applied. In fact, in order to guarantee correct operation, TFS 2010 will block all unpatched clients. You can get the GDR for 2008 here: Visual Studio Team System 2008 Service Pack 1 Forward Compatibility Update for Team Foundation Server 2010. Updates for 2005 and MSSCCI are coming later.
Read the detailed post “Compatibility Matrix for 2010 Beta 2 Team Foundation Server to Team Explorer 2008 and 2005” for all the info about the compatibility from different clients with TFS 2010.