posted a "BIOS and Kernel Developer’s Guide For AMD Family 10h Processors" and it talks about various features of the K10 architecture. One interesting part is where it mentions "up to two double-data rate 2 (DDR2) or 3 (DDR3) system memory DRAM interfaces.
It's pretty clear from this that AMD does indeed support both DDR2 and DDR3 memory in the same memory controller, even though we were sceptical at first when we heard about this.
We know that Intel aren't too keen on "combo" boards with both DDR2 and DDR3 as the performance isn't great for either memory type, even though it offers an upgrade path for the user.
MSI removed the DDR2 memory slots from its X38 board due to this and the question is if AMD is doing this purely for backwards compatibility or not.
So far there are no AMD compatible motherboards that supports DDR3 and we don't expect to see any all that soon. It's also more complicated to design a motherboard for DDR3 due to the shorter trace length compared to DDR2.
That said, we'd expect to see DDR3 boards emerging in due time as prices for DDR3 memory drops. At least AMD has been thinking forward this time and is already ready for DDR3 without the need of a new memory controller as per the move from DDR to DDR2.
You can read the entire PDF here