I'm probably way, way off here, butam I right in believing that the ExpressCard slot on the MacBook Pro will accept PCI Express cards? And if so, does that not mean we could plug in PCI Express graphics cards?
I am not sure about the details of the ExpressCard slot but I believe that it is essentially PCI-e. However, the form of the slot/pins is very, very, very different from a PCI-e cardslot and there is no way you could put a graphics card in there.
It is possible at a purely technical level to have a PCI-Express port (16x physical) card that would plug into the ExpressCard slot. The only problem is that ExpressCard only provides a single PCIe lane. This would mean that any card plugged into the 16x physical slot would be running at 1x electrical. This is way too slow for current graphics cards that require an 8x connection at the minimum, 16x preferred to run at a decent speed. Add to that then power requirements (most modern graphics cards need more power than you can draw through a 16x PCIe slot) and you will see why this is doomed to failure.