AMD had already said that AM3 CPUs would be compatible with AM2 and AM2+ boards but most manufacturers still haven't delivered on AMD's report. Not all of them, though: ASRock has recently announced support for even chipsets based on the aging nForce 3 and ATI 480X.
The list is big:
As for this list, some models are missing, most notably the whole line of GeForce 7000 chipsets, when the aging 6150SE with a 4Mb BIOS can run AM3 CPUs does.
One of my sources was able to tell me that the lack of support may be down to the combination of chipset used and size of the BIOS chip. All with 8Mb can work, some can work on 4Mbit, but a considerable amount of them with 4Mb BIOS can't.
The ASRock nForce 3 model (AM2NF3-VSTA) has a 4Mb(512KiB) BIOS, as we can see by the BIOS file:
Given the constand FUD surrounding compatibility questions, it would be nice if all of them just came out public on why some boards are fine for AM3 CPUs and others don't - or AMD for that matter (customer-centric innovation, anyone?) . I haven't done a through enough search but AM2 boards are the ones lacking more in the transition. For instance, ASUS is a little short, albeit supporting some GeForce 7000 models.
Last year, I had already reported something similar. Some AM2 motherboards with 4Mb BIOS are indeed able to run Phenom II.