Can Apple do even better this year? There are arguments on either side.
Stephen Baker of NPD makes the pessimistic case. He says the U.S. smartphone market is maturing, so it's getting harder for Apple to grow faster than the market.
Specifically, he argues that the carcasses of BlackBerry and Symbian have nearly been picked clean at this point — to gain a lot more market share, Apple will have to win converts from stronger competitors like Samsung. (Read More: Apple's iPhone Needs to Dazzle as Market Gets Crowded )
He makes a logical argument. But it's important not to push it too far.
For starters, Baker is talking specifically about the U.S. market, when much of the iPhone's growth has been coming from Asia (and China in particular). (Read More:Apple's iPhone 5 Sales Could Add Half a Point to GDP )
Then there's the matter of upgrades: Two years ago, the iPhone 4 was a big hit. In the last two quarters of 2010, Apple shipped 30 million units. Many of the customers who bought those phones will be eligible for upgrades right around now — and if Apple can get a good number of those 30 million units to upgrade to another iPhone, that's a potentially big boost.
And then there's the dual incentive of a new iPhone design and the first device with built-in 4G LTE.
If Apple plays its cards right, that combo could make the new iPhone the ultimate vanity phone: its long profile will make it instantly recognizable as the latest version, and LTE will make it perform far more quickly than previous iPhones.
The biggest factor determining whether iPhone numbers continue their upward march? It might be the progress of talks with China Mobile, the biggest carrier in Apple's fastest-growing market.
If Apple can finally ink a deal with China Mobile to sell the new iPhone, the impact of adding that carrier could dwarf all other factors — good or bad — influencing Apple's iPhone launch.