Re: The 2-iPhone craze
Actually, 2 iPhones each would drive data usage quite a bit. What uses the data? Updates. I've done the legwork on this, and app updates + OS updates account for something like 40% of data consumed. Increasing dramatically is browser usage, as people pull down higher and higher res images while browsing. Oddly enough, streaming isn't that big, as most people seem to only do that while at home/work where there is Wifi.
The updates eat the data plan, I suspect, because they trigger on a time basis, not a "what am I connected to" basis. So if the update trigger goes while you're on the bus, then the apps auto-update. Also, there's a bizarre thing in my studies where people pulling OTA OS updates seem to do it first thing in the morning while waiting for the bus. They get bored, accept the update then fidget ceaselessly whilst waiting for the bus to arrive.
Of course, my sample size is something like 50 units across 40 people, so consume the above with heaps of NaCl.