The question about the tablet form factor is interesting. The OS and UI has very intentionally been designed to scale to many different sizes and forms. If you pop by our office you might have a chance to see it running on a tablet :-). We chose this form because of its mobility and the ability to use it neatly as a trackpad. Plus we wanted to avoid the 'yet another tablet' syndrome but bring out something really iconic and different.
On a related theme, I don't actually see desktop computing disappearing anywhere. Sure, people use tablets on the sofa, but for work, a keyboard, and an 'indirect manipulation device' (=a mouse or trackpad) is highly desirable for purely ergonomic reasons. I was once at a presentation by a 'visionary' talking about how pure mobile was the future and computers were going to die. Every single person in that room was sitting behind a laptop. There's a reason for that.