... iPhone novelty factor probably didn't hurt regarding creating a big network load, as most iPhone users happen to have just bought/received their device in the last few weeks - quite often as Christmas present, and over Christmas/New Year actually have had time to play around with their new gadgets and explore what they can do with them ...

... which is also a plausible explanation why traffic dropped again somewhat after this 'exploration' period - under 'daily-life' conditions and maybe more in-line with other smartphone user patterns, while growing number of devices in the field will drive a sustainable increase in traffic.