I have had problems with this error (also in Visual Studio 2010) even when I don't have the .rc file / resource.h file open.
The only way to fix this for me was to manually open both files in the text editor (sometimes I get a msg box "The document '[path]\resource.h' is already open. Do you want to close it? which I answer with Yes), close both the files and try to open the resource file in the resource view.
This MS Connect page suggests closing the solution, deleting the .suo file and reopening the solution.