Working with Python 2.6.5, I find I cannot put multiple CABs in the same installer.  This is due to this statement in msilib.CAB.commit():

        add_data(db, "Media",
                [(1, self.index, None, "#"+self.name, None, None)])

The key, 1, must be different for each record in the 'Media' table.  The symptom is an exception something like this:

_msi.MSIError: Could not insert [(1, 3, None, '#foo', None, None)] into Media