A few observations:

1. Dennis solution involves hardcoding of the Workbooks>>??

2. Mike 
"Dim WithEvents m_cbButton As Office.CommandBarButton"
will fail miserably for users of older versions: ie 97

3.if the user resets the toolbars after Workbook is opened you will get errors with :
Private Sub Workbook_Activate() 
    m_cbButton.Visible = True 
End Sub 
 
Private Sub Workbook_Deactivate() 
    m_cbButton.Visible = False 
End Sub
(I only say as from experience!).

4. Mike with your code only the creating Workbook can run the code:
If ThisWorkbook Is ActiveWorkbook Then
(all subsequent books will fail?)

5. Personally I would make an Addin for the control and check for it's existence in the Workbook_Activate() / Workbook_Deactivate() events,
I would place all the code in the addin to do enable, delete etc.

6. If the user also delete your custom control or move it to another toolbar you can check that with addin code.

Maybe am being a bit pedantic, please feel free to shoot me down in flames