Greetings,
done everything suggested without any hope. I knew something must be wrong. Upon carefully checking up of my homemade shield I realized that the ironical irony acted again. I had a minuscule (literally) bridge between MOSI and GND. Too bad for me !
Now it works !
No need for a pre-defined MAC address. It can take any generic one.
I have my reef tank controller (Arduino Mega) broadcasting its status using Wiznet 811MJ connected to a wrt54g in bridge mode. Pretty nice setup.
I noticed though on my tests that the Wiznet needs a hard reset after power resume. I will set up a digital pin to do this. BTW. Is there any hardware option available to this ?
Thanks a lot to everyone,
Marian