If you want to save yourself some hassle of looking and buying the SIM yourself, there's a seller on eBay selling various prepaid German SIM's. I bought one from him for my trip. I'm mainly interested on having data access on my phone and he suggested a blau.de SIM with 10 euro preloaded. The selling price was 29 USD shipping included.
Before I left I selected and activated the package (online via blau.de website). Then yesterday when I got on the S bahn from the airport, I put the SIM in my phone and voila, I have data access.
I can't remember the seller name, but if you're interested I can look it up.