In a decision from May 2004 a district court in Munich ruled that the GPL is in principle legally effective. A developer of the netfilter project sued the company Sitecom, which uses the netfilter software in at least two of their routers without keeping the conditions of the GPL. The decision isn't effectual yet, because Sitecom still has the possibility to file an appeal.
The opinion of the court (PDF, in German) can be downloaded from the lawyer's page:
h**p://www.jbb.de/html/?page=news&id=32
They also promised to translate this document soon.