I've been using a SIM from Vodafone.DE in my unlocked iPhone 4S (as well as in my Android phone) using the Prepaid CallYa Smartphone Fun S tariff while in Germany. I toggle on a flat rate data tariff when I'm there to get some fast data and then it slows down after some usage to slower data for the rest of a month. Easy to get top up codes at many places, only had to show my Passport when getting the SIM at the store the first time.