I recently discovered that after you install the 2008 Team Explorer (here) - and you already had the 2005 Team Explorer installed - you may not able to view or edit TFS work items in Excel. Instead, you get an error that says: "TF80076: The data in the work item is not valid or you do not have permissions to modify the data. Please correct the problem and retry."
This error is caused when two different versions of the TFS Office Integration plug-in are installed but not configured properly. More details of the problem and a work around can be found here. However, the instructions for configuring your machine to use the 2008 add-in aren't correct. I've posted the correct details below. Make sure you close all instances of Visual Studio prior to following these steps.
To use the Visual Studio 2008 Team Explorer add-in
1. At the command prompt, change directories to:
C:\Program Files\Microsoft Visual Studio 8\Common7\IDE\PrivateAssemblies\
2. Unregister the Visual Studio 2005 Team Explorer add-in by running the following command:
regsvr32 /u TFSOfficeAdd-in.dll
3. At the command prompt, change directories to:
C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\PrivateAssemblies\
4. Register the Visual Studio 2008 Team Explorer add-in by running the following command:
regsvr32 TFSOfficeAdd-in.dll