Hi 4jochen !
Who will adopt the driver first?
If someone adapt 5100 driver for 5200, I could be the first to adopt it !! 
More serious : Cortex M3 working code helps 5200 data sheet hands on...
Driver is in this project :
http://www.wiznet.co.kr/UpLoad_Files/ReferenceFiles/W5200_TCP_and_UDP_SampleCode_v1.0_kr.zip waiting for AVR/Arduino porting .....
Any idea about (approximative) unit price (hobbyst) for WIZ820io ? about 10€ if chip is 3 to 4 € (reply #23)
The W5200 is a 3 to 4 Euro chip (3in1: HW TCP/IP + MAC + PHY)