Q:

What voltage is put across the Tx+, Tx-, Rx+, and Rx- lines over ethernet?

I'm trying to figure out how to wire up an LED, probably controlled by a FET, for an activity indicator for ethernet. I'm not trying to make one that supports official error codes or anything, but just something that blinks on and off as the line goes high or low. 
So what is the voltage differential applied to the lines, and would they be high enough to power an LED or control a FET spliced into the line?
EDIT:The point of this indicator is actually not to verify transmission or proper TCP/IP activity, but instead just to make sure that there is anything happening on the line.

A:

This isn't practical. Since modern Ethernet has a code, there are transitions even when there are no packets. The LEDs you see on networking equipment are driven by the PHY, not straight off the cable.
Even given that, the transitions are way too fast to see it go off an on with tranisions. The light would be apparently always on, and not blinking.
If you wish to wire up a signal, the fast Ethernet (100BASE-TX) voltages are +1 V, 0 V, and -1 V. I believe it is ~100 ohms. 1 V/100 ohms = 10 mA. If your LED is in parallel, you need it to not effect this too much.

