being able to see all incoming and out going bandwidth usage, maybe even be able to break it down to packet type reporting (x% TCP y% UPD z% Other)
this is exactly what I'm working on. I don't know if it is possible (processing power) but if I could just -depending on the type of packet- send different serial commands to the arduino that would already be a great step forward. also srcport, destport and possibly other values can be interesting information.
Luca: by going analog you would lose a great deal of potentially interesting information don't you think? also noise could give you lots of false alarms.
I'm gonna try some thing with all the info earlier in this post. I have the wiz5100 ethernet-shield btw