I "had" the very same problem during the NAB show in Las Vegas last week.

It is not hardware - after a call to Apple Care, they suggested I go to to the Apple store ( I also have ProCare ) - and it was software. After they were able to determine that booting up on another HD got the ethernet to pull up an IP address, they did an "archive install" or reinstall of the OS, w/o losing any data, just passwords. ( it took about an hour ) my ethernet was up and running at the store a short time later.. ^