While interesting, I don't think I would deduce that smart phones are, in any way, "replacing" computers.
Smart phones are probably replaced much more often than a PC simply because of the additional abuse they take. They're tossed around, carried everywhere, every day, dropped, etc. Even the average laptop has it much easier. On top of that, more and more smart phone choices are available now than when that market was younger (not only physical devices, but different OS' too). I don't know the last time I saw someone with a new non-smart phone.
I also have to wonder what their numbers for "Personal Computers" consist of. Meaning, are they only counting pre-built machines? I'm sure there's many like myself (especially here on /. ) who buy separate hardware and build their own computers, which is much harder to account for in terms of computer sales.