When the Forward Compatibility GDR for Visual Studio 2008 shipped, a change was made to the Team Foundation Server 2010 code that blocks incompatible clients. That means that after TFS2010 beta2, clients that don’t have a forward-compatible patch installed, won’t be able to connect.
For more information on what features will & won’t be available, see the TFS2010 Compatibility Matrix.
When a client connects to TFS, the UserAgent is checked against a list in the TFS registry and if it matches any of these, an error message specific to that client is returned to the user. For example:
Team Foundation services are not available from the server.
Technical information (for administrator):
TF253022: You must update your client with the Forward Compatibility Update in order to connect to the Team Foundation Server that you selected. To obtain this update, go to the Microsoft Web site: http://go.microsoft.com/fwlink/?LinkId=16648
The list of UserAgent’s that is checked is defined in the \Configuration\Application\DisabledUserAgents path in the TFS registry of the instance:
Team Foundation (*.exe, 8.0.50727.<4406) – Visual Studio 2005 without the forward-compat GDR (available closer to launch)
Team Foundation (*.exe, 9.0.21022.*) – Visual Studio 2008 RTM
Team Foundation (*.exe, 9.0.30729.<4172) – Visual Studio 2008 SP1 without the forward-compat GDR
Team Foundation (VSSConverter.exe, 8.*)
Team Foundation (VSSConverter.exe, 9.*)
Teamprise*/3.<3* – Teamprise 3.3 introduced basic support for TFS2010
Of course, if you know how to change the TFS registry, you can remove these bans. However, I wouldn’t recommend doing that, because using incompatible clients against the server can cause some strange things to happen. You’re much better off just patching the clients.