BrokenThought, I loves you man! Deleting the two files worked. And this is after talking to several Apple support techs the last three weeks who was puzzled by the issue. I think a usb modem I used earlier may have caused the problem (which the Apple people didn't know - gotta give them that), but don't know for sure. Anyway it worked and I'm a happy pig again
The idea of deleting (hard drive)>Library>Preferences>SystemConfiguration> com.apple.network.eapolclient.configuration.plist + NetworkInterfaces.plist works, sorta; but there's an important followup step that nobody's mentioned.
When I follow these steps (go to /Lib/Pref/SysConfig/, trash/rename network config .plists, restart), the result is that all my physical network interfaces are disabled: Ethernet, Airport, USB, FireWire, Thunderbolt, Bluetooth. And you can't turn them back on just by reconfiguring and applying the new settings. The trick I found to rectify this was to:
A convenience is that even though the old physical interfaces are crippled, you can still see those services before you delete them, so you can record the previously functional settings and attach them to the "new" service.
Deleting the two .plists does reset the BSD names of the physical interfaces (E-net and Wi-Fi cards), but it seems that the network services are attached to the BSD names at the Darwin level, rather than to the visible names in the OS X GUI. You have to kill off the software connection to the old BSD name (Ethernet=en3) in the Network system preference before a connection to the new BSD name (Ethernet=en0) can be established.
Since nobody's actually mentioned hardware in this discussion, let me get mine on the record. What worked for me may or may not work for everyone; automatic configuration on startup sure didn't work.
Early 2011 13" MacBook Pro i7/8GB RAM/500GB HD
OS X 10.9.5