Hello,
I'm not an ethernet expert... I don't think i can help you a lot...
netnut:
Are any of these calls blocking:
client.available(), client.connect() or client.connected() ?
I think no: during my tests, I removed all the "reading calls" on the arduino, ignoring any packet coming from the other device; this didn' change anything :~
Someone told me that it's some code inside the w5100 that is defective and blocks the ethernet card. The arduino is still "alive", because it can test the link and reset if necessary...
good luck!
and if you find a clue, please tell us 