Aright just to clarify and recap, and bundle some of the correct things that's been said in here in one easy to understand little sentence.
Socket A = the north bridge had the memory controller and was dual channel capable.
Socket 754 = a 64bit CPU compatible board, the memory controllers are on the CPU themselves, and the memory controller was not set up to run dual channel
Socket 939 = again, 64bit CPU with memory controller on the CPU and it WAS made to do the dual channel.
As for the whole "AMD wanted people to buy 940 over socket 754 and 939 wasn't out yet" that's not true.
It was known from the beginning that 940 was server boards, but also some may not know the FX series originally started out on 940 and later moved to 939, but that really has nothing to do with what's being said.