You don't need a German bank account and you can register with any existing German address - they do prove if the given address exists, but not if you actually live there. So foreign visitors can flawlessly register a Fonic SIM.
Don't forget to provide some other Fonic customer's number in the "Mobilfunknummer/Aktionscode" field upon registration, which will give € 5 of extra credit to you and the other customer.
For further details on Fonic check these two links:
Germany - Fonic - Prepaid Wireless Internet Access
FONIC ? Prepaid-Wiki