Why is Microsoft Surface horizontal?

Let's talk about form factor, and what impact it has on users when working with Microsoft Surface. Microsoft has said we expect every surface in every room will be a computer someday. Why is Microsoft Surface horizontal then? The video below, part 3 of the 5 part series, I talk with Stimulant about the horizontal form factor and various merits including how it promotes collaborative computing. At Microsoft Surface, we think that's a very important departure from how you work with your computer today. When you think of Surface, think beyond just it being a multi-touch device.
-Eric