Ahh.

You're right.

The problem is that there has to be an active/visible workbook to change the
calculation mode.

This may work better (not vigorously tested):

Option Explicit
Private WithEvents XLApp As Excel.Application
Private Sub Workbook_Open()
   Set XLApp = Excel.Application
End Sub
Private Sub XLApp_NewWorkbook(ByVal Wb As Workbook)
   ChangeTheCalculationMode
End Sub
Private Sub XLApp_WorkbookOpen(ByVal Wb As Workbook)
   ChangeTheCalculationMode
End Sub
Sub ChangeTheCalculationMode()
   Dim TempWkbk As Workbook
   If ActiveWorkbook Is Nothing Then
       Application.EnableEvents = False
       Set TempWkbk = Workbooks.Add(1)
       Application.EnableEvents = True
   End If
   XLApp.Calculation = xlCalculationAutomatic
   XLApp.Iteration = True
   XLApp.MaxIterations = 9999
   If TempWkbk Is Nothing Then
       'do nothing
   Else
       TempWkbk.Close savechanges:=False
   End If
End Sub

This seemed to work ok in xl2003.