Onboard Memory Controller

I figgure this is a good one for Wes, Ben, or the General

Since the memory controller is onboard the chips (the amd ones that is, not
sure about xeon)..
But since the memory controller is onboard the processor itself, does that
limit the processor to only one type of memory (like ddr) or can the
motherboard makers choose to build systems that use DDR2 or "other" stuff.

Just wondering is all..
well.. more like it's going to affect a buying decision within the next few
months