You may have read before about the fact that the TFS protocol version number changed between beta 3 and RC. As a result, you have to upgrade pre-RC clients when you upgrade the server to RC or RTM (the version number for RTM should be the same as RC).
The other thing you'll need to update is any web services that you have subscribed to TFS event notifications. For example, I updated the Continuous Integration example code from Doug's TechEd presentation.
In your code, you'll need to change the "02" to "03" in the SoapDocumentMethod attributes.
RequestNamespace = "http://schemas.microsoft.com/TeamFoundation/2005/06/Services/Notification/02")]
New (RC and RTM):
RequestNamespace = "http://schemas.microsoft.com/TeamFoundation/2005/06/Services/Notification/03")]