I have a theory, and it's really just my own speculation on why they reduced the footprint of the keyboard.
There's been a lot of chatter the last 2 days about Apple ordering 10in touch displays, which most are touting as the sign of a tablet or netbook. I am going to take a different tact on this based on some of the product information from the company they bought to acquire the touch software used in the iPhone and the new trackpads.
Perhaps, Apple plans to move away from the standard mouse as we know it, and roll out some slick, multi touch device that can be configured, and dynamically configured based on task, etc..
Totally speculation, but with a larger "touch" surface device I could see the desire for a smaller keyboard.