^no, it's more like nVidia won't allow SLI on non SLI certified boards. They only allow certifications on the newer intel based boards because they have to as they don't have chipsets for them.
The 750a and 780a (renamed the 980a recently) chipsets support AM3 and DDR3, but only MSI is currently making boards with those chipsets. NVIDIA isn't pushing their AMD chipsets much, and they aren't allowing SLI certification for AMDs own 700 family of chipsets, so that's why it is the way it is. Anyway, there are plenty of loaded full ATX AM3 Crossfire boards, but if you're into MicroATX that's mostly for the budget and HTPC crowd so the motherboards are tailored accordingly.