Microsoft has reported that this problem will be fixed:

 ” ‘Object must be the same type as the enum’ exception calling CommandBars.Add from an add-in”
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=462766

Notice that the “Embed Interop Types” property seems to have been renamed to “Isolated” in VS 2010 Beta 2, in one of those renaming changes that make things more obscure…