If I recall correctly, AMD's AM2 platform caused DDR2 to get significatly more affordable. hopefully, history will repeat itself with DDR3
If AMD's latest server roadmaps are any indication, multi-socket Opterons won't start supporting DDR3 memory until 2010 with the 6-/12-core Sao Paulo and Magny-Cours processors. That said, two separate reports hint that AMD will add DDR3 support to its Phenom desktop CPUs a little sooner than that.
CHW claims to have nabbed an AMD desktop chipset roadmap, and that roadmap explicitly mentions DDR3 memory for products due in the late fourth quarter of 2008 and early 2009. Corroborating that information, Fudzilla says the memory controller in AMD's 45nm Phenom core—code-named Deneb—will indeed support both DDR2 and DDR3 memory.
You may not want to count on any future Phenom motherboards featuring both DDR2 and DDR3 slots, though. According to Fudzilla, DDR3 support will involve a new socket (AM3) with a different pin layout and package than the current AM2/AM2+ socket. On the Intel front, leaked roadmaps suggest desktop variants of Intel's next-generation Nehalem processors will only support DDR3 RAM.