Many thanks to Ulli Exner, Esther Hintersberger, Mark Handy, Mark Jessell, Neil Mancktelow, Hugh Rice, Helmuth Sùlva and Gerhard Wiesmayr for fruitful discussions. This work was supported by the Austrian Hochschuljubil¹umsstiftung der Stadt Wien (Project Nr. 166/2000) and the FWF Fonds zur Fùrderung der wissenschaftlichen Forschung (Project Nr. P13227 and P-14129-GEO).