socket 939/940 RAM

Yo dudes,
I have a quick question here. I know that Socket 940 processors require registered ram and 939 don't. My question is this: is the registered ram a requirement of the processor or the motherboard? Furthermore, since 940 boards (according to AMD) will be able to run 939 processors, will I be able to use my unregistered ram on my socket 940 board?
thanks for the help.

Tom

I believe it is a requirement of the memory controller, so in the case of the opteron, the cpu. In the case of just about eberything else, the motherboard.