Looking at the specs for the Arduino Ethernet, it seems that it has everything you would need (hardware wise) do packet captures right from a span port. Being new to the world of the Arduino, is there alrady code out their somewhere that will capture the ethernet packets (like wireshark or TCPDUMP) and store them on the available SD card?
Can't remember seeing such thing, think you need to patch the ethernet library with some additional functions for that.
Would be a challenging project - not for the faint of heart 