So it looks like that no recent chipsets from ether AMD nor Intel actually have PCI support (Z77, Z88, 890FX, and 990FX) Which would mean that any motherboard that uses these chipsets and also has a PCI slot needs to have some kind of PCIe to PCI conversion on the motherboard itself - So in theory a PCIe 1x to PCI adaptor card should work just as well as a PCI slot on any motherboard that uses the Z77, Z88, 890FX, or 990FX chipset
On a further note the Wiki articel on PCIe talks a bit about PCIe backwards compatibility. But only about running PCIe cards in a PCI based system:
Well there is the weird realm of PCI to PCIe 1x adaptors, but quite frankly i don't even know how well they work, and it would look like a bit of a bodge job :(
