Yes, for AMD systems, dual channel is only supported on the socket 939 CPUs and SOME of the socket A motherboards. Intel is a different story, but here I am only talking about AMD.
It wasn't not so long ago when i owned socket A nforce2 ultra dualchanel memory board (probaly every hard-core gamer has owend one) with AMD XP 2400+. It was gigabyte 7N400-L