On a yearly basis, iPhone has been growing at 270% in 2008,83% in 2009 and 130% so far this year. The growth has been faster than the growth of the smartphone market during the same time. As a result, the market share of the iPhone has increased from 2% in 2007 to 13% in 2009.
The question now is: What is the required growth rate for iPhone to maintain growth in share as the overall market grows?
Taking market estimates from Morgan Stanley, I tried to fit an iPhone growth rate which would result in a 20% share for Apple by 2013. This rate turns out to be a conservative 50%/yr. Any growth beyond 50% would result in well over 20% share for the iPhone.