Exact same issue with my build (Cross Hair V Formula). Contacted ASUS their response was the same as you got. I've look around the internet. Some seem to have a solution but I'll be darn if I can figure out what it is. I'm about reinstall Win 7 Pro, I will also try Win 8.
Router tested on all ports by known good G73JH - Passed
Cat 5 cable tested - Passed
Installed drivers Intel Gigabet 18_7, V16400,V16500, ProDOS, ProWin64 and ProWinx64 with the same result.
OS cannot find Intel Adapter (believe it to be Intel 82583) Asus cannot confirm what was integrated into the motherboard while having the serial number????
Device Manager does not show any network device.
System Info shows no device
IPconfig /all shows no device
Cannot find a MAC Address
The latest ASUS BIOS 1703 is installed
Did you find a fix or have to send the board back?