I got a new MacBook pro, one of the core i5 ones, and transferred all of my data with time machine restore, which is absolutely awesome, btw. But my ethernet interface never worked! Being the IT guy I need my ethernet interface to work, so I searched online for someone else who had the same problem. Unfortunately, everything I found was simple fixes for simple problems. So I made an appointment with the mac genius hoping they could solve my problem, and they did.

They went to my hard drive->Library->Preferences and deleted the folder there called “SystemConfiguration”
Then rebooted, and viola, a working ethernet interface.

So, what must have happened to break mine, is when I restored my mac with time machine, it confused the network interfaces somewhere. The interface showed up, but always said ‘cable unplugged’. I had tried everything; deleting the interface in the network section of system preferences, configuring the interface with ifconfig in Terminal, creating multiple interfaces, rebooting, different cables, different networks, and different locations. The time machine had restored my network config folder and prevented OS X from correctly seeing the right interface.

The Mac genius said it can also help if you’re having problems staying connected to a wifi network.

Hope this helps!

Update 6/7/2010:

Here is a good article on troubleshooting your Mac OS X wireless networking at osxdaily.com