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Last I checked, there are ~1.3 billion PCs for ~2 billion PC users - a little less than "everyone and their dog and their dog's squeaky toy" or "Computers are everywhere" (and people with 2-3 PCs of their own are nowhere near average ...but I suppose such whims are what brings this http://en.wikipedia.org/wiki/File:Human_welfare_and_ecological_foot... insanity, and generally resulting in http://en.wikipedia.org/wiki/Planetary_boundaries )
Meanwhile, there are more than 5 billion mobile subscribers...
Whatever the reasons for those differences (costs being of course a large one, but in a broad sense: "even the poorest person" absolutely can't easily have a P4 PC, if only because of the cost of electricity - not in a 'how much a kWh will cost me?' way, more like 'what kind of fortune for bringing a semi-reliable mains electricity to my home?'), large part of humanity is clearly more receptive to smallish, relatively inexpensive, mobile, battery-powered devices. And I suppose that large Android phones (but without the silly price premiums such models command now in the ~west) might become a dominating form of ~tablets of sorts - or "personal computer" (hey, we reinvented what that means few times already, really) - in the next decade or so; hardly a niche.
(also http://www.opera.com/smw/2012/03/ "Connecting the unconnected" section)