The introduction of the Express Card slot changes this. This is because inside the connector two types of interfaces are available: USB2 and a PCI-Express 1x bus.
Seizing this opportunity, Asus is offering what they call "the world's first external graphics card station for notebook computers": the XG Station
Once connected to a laptop computer via the Express Card port, this external station hosts a regular PCI Express slot, allowing the use of any video card, then plugged to one or two displays. It also hosts two USB 2 connectors, and a 5.1 audio output. Technically, if its Express Card connector is the 34 format, nothing should prevent that product to operate properly on a MacBook pro, which could then run not one but 3 external screens.
Of course, the 1x slot won't allow the card to express its full potential, but as long as Mac OS X includes the necessary driver, it should work.
One serious issue remains: this product is announced with the very hefty price of 600$. It would probably be desirable that another manufacturer offers a simpler version, with less features and only allowing to plug any PCI-Express card on a laptop. Imagine the productivity gain one could make by plugging a S-ATA II card and a few disks in a RAID array on a laptop.
