It only solves one side of man machine interaction: the machine-to-man part. For many interactions, its solution for man-to-machine is voice, which is anything but unobtrusive in public.
As long as the interface makes the number of choices available at each "page" of the interface pretty low, then input devices get small and easy to hide.
http://library.naist.jp/dspace/bitstream/10061/7966/1/EUROSP...
Another gadget to wear, but better design and miniaturization can help.