I have been having a mess of a time with multihoming on my 2011 Apple MacBook Pro since I have moved to Mountain Lion (10.8.x) and found all sorts of others in the Apple Support Forums with wake from sleep ethernet issues and other similar problems. The ethernet port recognizes there is connectivity but doesn’t accept a DHCP address.
I was right about to rip-out all my network-related .plists in /Library/Preferences/SystemConfiguration when I thought that it might be getting confused with DHCP if I have turned-on Internet Sharing in the Sharing Control Panel. Sure enough, I had it on for some reason doing some network work a few weeks ago and that was the source of the issue.
After disabling that, I was able to multihome again without a problem.