The largest growth factor is the iPhone's expansion to new carriers, most notably Verizon in the US in 2011, when their exclusivity deal with AT&T runs out.
In a different note, RBC Capital Markets iPhone shipments will increase to over 82 million by 2012, enough to take an almost 20 percent market share of the smartphone market.
"Despite the iPhone’s introduction more than two years ago, competitors continue to lag iPhone’s sleek touchscreen experience, robust third-party applications platform, and tight multimedia integration," Mike Abramsky of RBC says, via Apple Insider. "Off the strength of its brand, innovation, and customer loyalty, Apple is expected to retain its premium carrier subsidy versus competitors, sustaining above-peer margins for its smartphones. We believe Apple will sustain its lead in content, games, and apps, which we expect to expand to include mobile commerce, user-generated content, advanced gaming, etc."