When your ethernet stops working all of sudden for no reason, make searching the Apple support forums (via some other way to access the internet if you can) an earlier step in your troubleshooting process.
Curse you, Apple, and your sneaky hidden update that broke my ethernet internet access. I wasted hours thinking it was a hardware problem. (Granted a couple false-negative results dragged that process out longer before I could eliminate every variable but the port. And even then, it's the software that's the problem!)
Fingers crossed that in cleaning the port, I didn't break it by mistake.
Also, Apple, please get on an update fix pronto! I'm not in any mood to reinstall my OS to fix your mistake and this didn't work.
10 years ago on TTaT: Compulsive behavior