Hi SurferTim,
The SD card thing never (well, I have the shield only a couple of days now) worked. But I will start fresh tomorrow using SD Formatter to reformat the card and your code. And if its no good, I might even go for another ethernet shield to buy myself the answer whether I am too stupid or just had a hardware problem. 
Thanks so far, I'll write an update if there are new developments ....
Christoph