A month ago ComScore reported that in October 2010 25% of Americans above the age of 13 used smartphones. The latest report shows that share to have risen to 26.3%.
That means there are 24 percentage points of penetration to go until the majority of Americans are smartphone users. In absolute numbers this implies about 56 million additional users (and hence units sold).
The rate of penetration growth was 1.3 percentage points per month. Assuming no acceleration in this figure and assuming we rely on ComScore (vs. Nielsen which reports higher figures) then majority share is 18 months away, or mid-2012.
However, assuming some acceleration and a different sampling method, I still believe that we could see this figure by end of this year.
In either case the tipping point is near.