There are some fairly strict rules about loading, capacitance, etc in UTP ethernet lines. It would be easy to get those out of spec so that you cause yourself some network problems. If you take this approach, make sure you monitor the error count (on both sides of the connection if possible) to see if your device causes problems.
Maybe another approach would be to use the ethernet-arduino combination to pull data from the router and monitor traffic that way. It does assume a correctly functioning router, but that's not a terribly unreasonable assumption.
Maybe your router can run dd-wrt, enable SNMP, and write some Arduino software to poll the router with SNMP. That would be a system with a lot of other potential uses.
good luck!
-j