Going straight to the source, I got a pretty enlightening answer. It seems that when the first batches of 45 nm processors were coming off the line, to get them validated they focused on DDR-2 and only ran the northbridge portion of the chip at 1800 GHz instead of the full 2 GHz that many were expecting (remember, the “uncore” parts of the 45 nm Phenom II run at 1.8 GHz while the processor cores run at 2.8 and 3.0 GHz). This allowed AMD to get these AM2+ parts out in relatively short order, and once those products were in the pipeline AMD concentrated on getting the DDR-3 parts out at full spec. So, the DDR-3 portion of the memory controller is still being officially validated, but from all indications it is working perfectly fine.
So far I am not under any NDA information, but from all indications it appears as though the DDR-3/AM3 launch is going to be sometime next month. Final products are ready to be sent out for review, and certainly motherboard guys like Asus and Gigabyte have put out big press releases about their upcoming AM3 based motherboads (and in Asus’ case, they have pre-announced for NVIDIA the existance of the nForce 980a chipset).
So, from all of my information at hand, there is no problem with the DDR-3 support on upcoming AM3 processors. They are already in production, and that very fact alone says to us that there is no problem with the controller at all. The one thing to really point out here is that the AM3 processors will fit in AM2+ boards without a problem (and the motherboards just need a BIOS update to run them correctly).
Availability on AM3 motherboards is up in the air right now, but as I had mentioned earlier, the big guys are already announcing them and we have some basic specifications (plus pretty pictures).