Well if it turns out (as is probable) that he knew about it since 2003, if not since 2002, he might well have to.
Which would be a disaster for SPD/Bündnis 90 as he really is one of the few popular politicians about. Joschka gets voters to the polls, he is a "Sympathieträger". He's just too important for the Red/Green coalition to lose, especially this near to a state election in NRW.
Spiegel article (in German)He isn't expected to resign though, Joschka wears his flaws pretty openly (he likes good wine and a wedding - usually his own) which makes him human. And he is likeable so people will forgive him a lot.