I believe that it may just be impossible. The graphics card on Apple's portable lineup is an integrated chipset. Unlike the desktop macs, which have a PCIe slot, you would have to have equipment to desolder the chip from the board. There are also may firmware difference and heat constraints you would have to worry about. The best you are going to get on there is the graphics card it came with. I wish there was a way to upgrade too, but with everything integrated in the mac it is impossible. I guess we will all have to wait to see what graphics chip Apple uses in the next gen.
Hope this Helps