Well, for the 17" and 20" model, the GPU itself is on the motherboard. MXM is a definition of a card the same way there are PCI, AGP, PCI Express, and even 'mini PCI', 'ExpressCard', and 'PC Card' slots. (The latter two fully enclosed to be easy to plugin/remove from the outside.) But, yeah, the 17" and 20" models aren't upgradeable, only the 24" model appears to have its video on a daughtercard. (The MXM card.)
Here's a comparison:
PCI slots were modified into the removable 'CardBus', and the less-removable 'mini PCI' (An AirPort Express card is really a mini PCI card.)
The PCI Express x16 Graphics slot was modified to become the MXM slot.
All three slots are electrically identical to their desktop counterparts, just with different physical connectors and card sizes optimized for mobile use.