Someone claiming here, the built-UMTS modem would work only with a SIM Mobilfunkabieters. This statement is Schlechtweg wrong. I (actually O2) tested it with a map of Tchibo and with T-Mobile and both went. A colleague uses a SIM by Vodafone and it goes well.
Nevertheless eyes on the "UMTS" purchase. Not with all UMTS access of all suppliers, it can easily surf the net.