There certainly is room for improvement in this area, with new protocols like Firewire and Serial ATA. I'm sure one day somebody will come along with a USB 4.x GeForce XPortable, which simply plugs into the USB port and takes over 3D graphics seamlessly.
One day, but not tomorrow.
The hard truth is that the only upgradeable components on a laptop are the memory (up to a point) and the hard disk (up to a point). For the rest, PCMCIA modules are mainly of the commmunication kind, although there are card readers and security systems (think biometrics) that are available.
And everything is expensive.
I have never heard of a PCMCIA-adaptable graphics card though. Without the link from pddxr, I would have said they don't exist.