The wrist watch metaphor is erroneous since most of these devices will not have screens. They will simply sit there and collect data, lots of it, like the fly on the wall. The data thus collected might be displayed on the screens of the larger form factors.
There was a wonderful article I read on the topic at ReadWriteWeb a few days back that I have not been able to dig up. But there is a similar post on AllThingsD.
Beyond Tablets: The Next Five Computing Form Factors