set objExcel = CreateObject("Excel.Application")
drPath = "serverfile"
if (objExcel.Workbooks.CanCheckOut(drPath) = True) then
    objExcel.Application.Workbooks.CheckOut drPath //note - may need to open first
    objExcel.Application.Workbooks.Open drPath
else
    msgbox("Unable to checkout SharePoint file: " & file.name & ". Please contact an administrator.")
end if

Private Sub Workbook_Open()

'The following command is equivalent to hitting the "Edit" bar
'that comes up when you open an Excel file by clicking on it in SharePoint,
'which is equivalent to "Edit in Microsoft Excel" (not CheckOut)

ActiveWorkbook.LockServerFile

End Sub