Paul, while it is quite feasible to make a mobo supporting both AGP and PCI-Xpress, making a translation connector would be quite a challenge. The bane of hardware engineering is connectors - whenever you bring a signal across a connector, you degrade the signal by adding trace length across a pressed fit, plus the uncertainty of environmental conditions (moisture, fingerprints, dust, etc.). Proposing an extra connector will send most hardware engineers screaming out of the room!
A video card plugged directly into an AGP connector can then be screwed to the chassis (via the backplane) and is a fairly stable connection. Once you add a PCI-X translation connector it is a problem to stabalize the board due to the added height.
Thus the video card vendors do not have to worry about people reusing AGP cards on their new PCI-X mobos!