Same problem:
Builin ethernet dont work on my PowerMac7,3(Power Macintosh G5 2.5 DP (PCI-X)) with clean installed MorphOS 3.10 on second HDD (600Gb SATA port 1).
Via DHCP eth0 dont receive IP, if use Static IP anyway connot ping any computer or router in LAN.

On same computer with same patchcord and same router ethernet work well in MacOS X 10.5.8 installed on first HDD (300Gb SATA port0), IP received via DHCP from router, I may browse computers in home lan and open any site in Safari.

Here bugreport file - bugreport.txt
Please help us.