I had the same problem...check your \Extra\com.apple.Boot.plist file. The following two lines should be in there:
<key>PCIRootUID=</key>
<string>1</string>
<key>EthernetBuiltIn</key>
<string>Yes</string>
If not add them, If you find the "PCIRootUID" key/value in the entires below REMOVE IT
<key>Kernel</key>
<string>mach_kernel</string>
<key>Kernel Flags</key>
<string>arch=i386 PCIRootUID=1</string>
Reboot and try again.
If that does not work you will have to go to \Library\Preferences\SystemConfiguration
In this directory remove/copy the NetworkInterfaces.plist to your desktop and reboot.
The problem is there is a mismatch in the device order of the network card. It should be "en0" and "BuiltIn". If that does not work, remove the file again then go to System Preferences and remove all network interface cards, reboot and add them back.
Hope this helps...this is not my fix but our realtek network guru/driver creator whose tag i do not wish to bastardize by spelling wrong.