Update for people from Germany:
I contacted Nokia-Club instead of Nokia Shop in Germany because my phone doesn't accept tone dialing atm, which is needed to go to the Nokia Shop (1=Club, 2=Shop, default=Club).

Anyway, a very friendly person helped me a lot to get some informations and he called his colleagues at the Nokia shop. After some minutes he was back and told me, that his colleague from Nokia Shop knows about Maemo and the 500 pieces developer discount, but it's unsure when the discount code will be activated in Germany and it's even unsure if this promotion/developer program from Maemo will be ever accepted in Germany.

... I'm feeling speechless