Mobile data in Germany? (but D1/D2 only)

[inquisitor]

Vlad is referring to the international WebSession, which will grant a volume of 50MB valid for 24h in most European countries, however only on one Vodafone partner network per country. Actually N24 and BILDmobil are both more expensive when it comes to the international WebSession, as they charge € 20 instead of € 15, but I don't consider any of these tariffs reasonable.
That prolongation-trick, which I described in the pwia-wiki, should also work with N24 and BILDmobil.

@Petkow
For the case you haven't noticed it yet, you have a personal message from me.

If you're looking for a long-term solution, there are also some interesting postpaid-tariffs without minimum term on T-Mobile and Vodafone:
UMTS Tarifvergleich | UMTS Flatrate Vergleich - Online Vergleichen und Sparen!
But those will pay off only if you stay with them for a longer period, because there's a setup fee ("Anschlußgebühr") of € 50-60.