I'm unsure whether this problem has affected anyone else, but with my Ethernet shield, when the Ethernet connection is disconnected then reconnected, I'm unable to access the Arduino anymore. I have to reset the Arduino to get another IP and then it's fine. Even if the Ethernet is disconnected momentarily, it still becomes inaccessible without a reset. This happens using the standard Ethernet examples, and the Ethernet library seems blissfully unaware that a disconnection has occurred.
Any ideas on how to detect it other than hacking a wire from the "link" LED to the Arduino? Mine's based on the Wiznet 5100 chip, so no link register. I could ping an address, or try accessing some webpage, but it seems like a real bodge.