Additional information: apparently no one involved in developing the Ethernet Library has tested communications with web servers other than Apache. The library as-is will not communicate with lighttpd. Phil's modification above does allow communication, you will need to take care to render each line into a complete string before trying to println() it; multiple print() commands per line will not work.
Hmmm, that's interesting. Is that standards compliant behaviour? I would have thought it didn't matter how the bytes arrived to the server, just that they did...
--Phil.