Ethernet does not work when plugged into the back of my new Thunderbolt display.

I can offer a pragmatic approach to making it work, though it's not really a "fix."

I summary, if I walk into my office and plug in my Thunderbolt monitor without opening my MacBook Air, odds are none of the attached USB devices or Ethernet will work. I last posted a reply saying that unplugging/plugging the Thunderbolt would fix the problem, but longer term it turned out to be inconsistent.

"Solution":

1. Open the laptop and wait for the screen to come up.
2. Plug in the Thunderbolt
3. Wait until the Thunderbolt screen begins to come up (blue at first), and close the laptop.

In my hands, all the devices connected to the Thunderbolt monitor now work every time.

It's a pain in the arse, but until Apple fixes the problem for real, it's a practical work-around.