There are a couple of boards that have support for both but 1 warns of potentially nuking your AGP card due tech spec differences between PCIe and AGP, so not to run 1 any longer than it takes to get a PCIe card, and preferably not at all. The other puts it on the PCI bus so the bandwidth is limited to PCI spec from what I read. Haven't heard of any others yet, and neither of those seems a good solution.
I have read Peter say they could just add the AMD 8151 chip for AGP support for the AMD boards, so maybe someone is going to do something like that?