MP3.de offers a prepaid SIM card with an option for a full Internet flatrate for € 29.95 a month. The offer uses the E-Plus network in Germany which means 3G coverage is limited to cities. HSPA is not yet available. Therefore, 3G connections are limited to 384 kbit/s for the moment. In non 3G areas, only GPRS (no EDGE) is available.

How to get the SIM
The SIM can only be bought via the online portal of MP3.de for €19.90 with €10 euros already loaded onto the card. Once it has arrived it has to be registered at the online portal before it can be used.

How to top up
On the online portal via a credit card (not sure if international cards work, has to be tested...) or via a "Bankeinzug" of a German banking account.

How to activate the Internet flatrate option
It seems the option has to be activated/deactivated via the MP3.de online portal

Settings
APN: gprs.gtcom.de
Username/password: not necessary
Proxy (should not be necessary): 195.230.105.23

Misc:
Report about the offer on Teltarif, September 2008