1000 thanks to SurferTim, CodingBadly and westfw. Now it works for me too.
@CodingBadly: Thanks for putting the arguments, why I didn't posted any assumed bugfix without getting any confirmation even on the bug itself. They exactly matches my reasons. Plus my assumed bugfix next day turned out to not really do it :-)
I found it. It was in the w5100.h file starting at line 253 in my version. The new compiler version did not like the old way.
I had (and still have) gcc 4.3.5, and experienced the bug
(I mean the one described in http://arduino.cc/forum/index.php/topic,57972.0.html
to avoid confusion). Now I only took your patch from the previous post (w5100.h) and now it works. I did not take earlier mentioned change in socket.cpp.
Even my keep-alive socket connect (for very fast sensor polling) is working now!
If anyone want to see the code (arduino code for the server and java for the client side) just ask for it. I just dont want to flood the board with code nobody wants to see.
Thanks again to you guys!