It's not impossible but I'd say it's unlikely. I dont' think there isn't any reason they couldn't use an agp to pci e bridge like nvidia did with some of thier exsisting cards on something like a 7800gtx but they beleive that the potential market is not large enough to warent doing it. If someone has 400+ dollars to spend on a video card it's not unreasonable to assume that they can afford a pci e mb or already have one. After all the market for 7800 gtx's is already a small one volume wise and a small percentage of that is virtually nothing. Besides if you bought a agp high end card now you'd end up either majorly limiting your selection for a motherboard next time you upgraded or have to get an pci e video card then anyway.
More mid end stuff like the exsisting x800's or the 6600's and 6800's will probably be around in agp versions for mid level upgraders for quite a while even if they are no longer sold on pcie. On the low end pcie seems to be taking hold quickly because it's primarily for new systems and it makes tricks using main memory, hypermemory etc, much more practical. Of corse there will continue to be agp cards in that range too but they will pretty much be older generation cards the 6600's or even older cards like ati 7200's. I don't expect any new designs in budget cards in agp anymore.