I think a lot of things are being missed here...
One is, Apple is attempting to reduce the foot print of all their products. People loved the quality of the previous cinema displays, but hated the base...enter the new AL displays. Smaller foot print, sleeker styling.
When Apple went from CRT to LCD on the iMac, they not only managed to create a machine that was a fraction the size and took up a tiny amount of desk space, but they created a machine that had functionality throughout the design. The display could be moved and adjusted as you wished. The question is, what is the evolution of this design?
Now, I don't know what that is, but, maybe its interchangable displays? Maybe, since the new AL displays are wall mountable, they have devised a way to snap either a 17" or 20" display onto the machine. It keeps people using the Apple displays and gives them the opportunity to upgrade their displays at a later date.
The hottest thing I've seen today was over at Spymac...
Check these out...
http://spinbox.techtracker.com/DA/6615/
http://spinbox.techtracker.com/DA/6614/
The first thing that comes to mind is, the opposite of your desk, would be your lap, so in that sense, this could be some sort of reference to a portable (I won't say PBG5, though I will scream if it happens). The other concept would be moving from the desk (work) to the living room (leisure). Maybe we're going to the wall, because the new iMac is wall mountable and comes with standard bluetooth keyboard and mouse?
Maybe I'm not going to sleep between now and keynote?