I got a problem with Linux kernel 2.6.18 and a nVidia nForce Ethernet: the MAC address is not recognized as valid by the kernel an thus a random generated MAC address is assigned:

forcedeth: using HIGHDMA
0000:00:0a.0: Invalid Mac address detected: 1d:9b:2a:38:1b:00
Please complain to your hardware vendor. Switching to a random MAC. 

I seems that in kernel 2.6.21 the issue is solved.