|"Is there some reason why PCI express cannot coexist with AGP?"|
It can. ECS sells a ECS 915P-A motherboard with pci express 16x and agp 8x along with pci express 1x and with DDR and DDR2.
GIGBYTE has a motherboard like ECS, but it has a G.E.A.R. slot instead of a full agp slot. You can put an some AGP's in a G.E.A.R. slot but it is only good for temporary use because GIGBYTE claims that it can shorten the life span of an AGP card.