Verizon has reported a rise from 2.7 to 3.8 million iPhones sold y/y. This means that there is still some growth in the US market, but it is special in that all carriers heavily subsidize.
The German market for example is much more price sensitive. While is the US a 2GByte plan with unlimited minutes sets you back a minimum of $60 prepaid or $100 postpaid with $200 for the iPhone 5 16GB, you can get such a plan in Germany for 40€ without subsidy or for 65€ with 29€ for the iPhone. German prices are all including a 19% sales tax, without it prices correspond to $45/m or $72/m + $33.
No sane person would be using a postpaid plan under this circumstances in the US, they do because of share everything. Extra phones come from additional lines, which cost you $40/m + $200 for the phone, which is a pretty good deal, and would account for much of smartphone demand in the US. Since there are no shared plans available without a phone subsidy, it means that for a large segment of the population they have no economic choice but go with device subsidies.
The important difference between the US and rest of the world is that devices cannot move between carriers. This means that carriers have a huge incentive to subsidize the phone to make switching less painful, and this has so far prevented the creation of separate markets for wireless service and phones, and has been the prime cause US carriers compete much less on price, and it also allowed Apple to cream off a lot from the carriers.
Now that we have seen the numbers for Apple, it gives another strong indication that the US is driving a lot of iPhone demand thanks to share everything: Americas revenues are up $1.6bn y/y, while iPhone revenues are up $2.3bn y/y. In addition, they said that US numbers increased by more than 50%. Assuming the rest remained flat, then US sales a year ago had to account for 40% of all iPhones sold to explain this, and this matches the current Americas share of revenue. Or looking at iPhone numbers, they grew 5.2m. If Verizon has 35% of the US, then its numbers indicate 3.2m added in the US alone.
The carriers are not making much money on extra phones on share everything: probably just $20/m after subsidies, and they have to provide unlimited minutes from them. This makes one wonder for how long they will want to continue to provide them.