The EFI string method worked for me, too, although I had to toy around a little bit as I'm already using EFI strings to get dual video cards working.
The steps I followed:
1. I did the steps mentioned in the first post of this thread, although it didn't appear to have any effect. I'm mentioning it here because I have no idea whether it was necessary to get the App Store working on my setup. My only internet connection on this computer is over wifi, and neither of my ethernet ports are plugged in.
2. Download EFI Studio (I used 1.1, a google search brings you to it pretty quickly) and gfxutil (from the aquamac boards-- again, google is your friend here).
3. Use EFI studio to generate an EFI string for Ethernet. Copy the plist text, not the hex string, and put it in TextEdit or something for safekeeping.
4. Copy existing EFI string hex code from com.apple.boot.plist (in my case I had it in the one in /Extra, in your case things might be different depending on how you have your system set up). Save into a text file.
5. Convert that text file from hex to a plaintext plist file using gfxutil.
Example:
a) open terminal
b) cd /folder/where/gfxutil/is/located
c) ./gfxutil -i hex -o xml ./graphicshex.txt ./graphicsplist.plist
6. Take plaintext EFI string for ethernet from earlier and add it to the graphics EFI string. I'm not sure whether it matters, but I put the ethernet string before the graphics string.
7. Convert new combined plist to hex with gfxutil:
./gfxutil -i xml -o hex ./combinedplist.plist ./combinedhex.txt
8. copy new hex string from combinedhex.txt (or whatever you named it) and paste into com.apple.boot.plist. Don't forget to put <key>device-properties</key> before it and to surround it with <string> </string> tags.
9. Reboot. You might also have to remove all network interfaces and delete NetworkInterfaces.plist too, but if I remember correctly I didn't have to.
