As best i read it you can mix some processors across the sockets, but not a good idea you may just give yourself a headache.
If you have different socket amd processors already and want to try them using the wikipedia article as a guide then try and see if they will work, but i would not buy a new cpu for a different socket motherboard.
It seems there is some level of backwards compatibility such as using an am3 cpu in an am2+ board, but not the other way round.