Joerg:

They generally like you to transfer the Copyright to FSF wich cannot be done as long as you write the contract on English because this language does not have the right words to express the real factual situation.

Once again, "copyright" and "contract" are not the same. Also, if you transferred copyright to FSF, it would be fought on the terms of US law, as that's where the publisher (ie the FSF) would be based. Finally, if you attempt to defend the GPL in court and fail, it would only cause a fall-back to the standard copyright law, thanks to the self-destruct clause in the licence. I doubt that day will ever come.

No, you may not be able to force people to release their modifications, but you can stop them using your copyrighted work against your stated will.