Westfw,
Interesting you mentioned this. Going analog was my original plan but then I got a digital oscilloscope connected to an Ethernet line and I couldn't really tell the amount of traffic just by looking at the wave.
I learned that 100Mbps uses a three-levels encoding -1, 0, +1 and even with no traffic the signal would still constantly change within those three values to provide clock synchronization.
http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/4b5b.html