Here is a little tutorial to get it working again. Please note, I currently have the Gigabyte P35-DS3P Motherboard so I can't guarantee this will work for you. But most likely if you have one of the supported Ethernet Controllers then it should work. I take no responsibility if it screws up your ethernet.
Lets get started...
1. First Download the RealtekR1000.kext from here (I used the Binary Form): http://store.psystar.com/opensource/r1000
2. Move the Kext in to /System/Library/Extensions and set Permissions (You should know how to do that )
3. Backup your IONetworkingFamily.kext from /System/Library/Extensions to somewhere safe
4. Run the following command only if you get conflicts or the RealtekR1000.kext fails to load...
rm -R /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleRTL8139Ethernet.kext
5. Restart your Computer and hope everything went well