Microsoft fixing bug "Standard user unable to load/unload COM-based add-ins registered for all users with the VS Add-in Manager on Windows Vista"
Two years after I reported it, it seems that MS finally understood that this was a bug and I have been notified today that it is no longer closed as "By design" but as "Fixed":
Standard user unable to load/unload COM-based add-ins registered for all users with the VS 2008 Add-in Manager on Windows Vista
This bug has plagued all of us developing add-ins with COM-registration (rather than .AddIn XML registration) for all users (not for current user) for backwards compatibility with VS.NET 2002 and/or 2003. Too bad that the fix comes so late that I doubt there will be a fix for VS 2008, only for VS 2010...but anyway, it will make VS 2010 a better product.