I have been hassling with this issue for the last week or two. However this morning I got it to work. I figured that it must be a problem with the system serial number in SMBIOS.plist so I merely generated a new one by using Multibeast and reinstalling the Mac Pro 3,1 SMBIOS.plist.
Voila.
This is confusing because all the talk on here suggests it's something to do with the en(x) designation of my network adapters. I am using a Ralink USB WIFI dongle and even when I deleted the adapters in Network Preferences and delete NetworkConfiguration.plist it doesn't change a thing. I then reinstalled hnak's intel 1000e kext and still nothing changed.
What's strange now is that my WIFI dongle is designated as WLAN and not en0 and everything works fine.
Weird little bug.
Cheers