Ok, adapting the built in PCI interface to AGP involves some signal and voltage switching and is definately possible. Gigabyte is doing this on one of their motherboards, the GA-8I915P Duo-A and they are calling a G.E.A.R. slot.
And on page 17 of it's manual it warns that it's a temporary solution until PCI Express X 16 cards become available because it may shorten the life of AGP cards.
Which raises the question... so did they just adapt the AGP port to the PCI buss like Gigabyte or did they do it separate from the built in PCI controller? If they just added the AGP to the PCI buss controller it might not be a good thing.