One of the more interesting graphs produced by former Nokia analyst Horace Dediu is up today. Horace is looking at how quickly consumers bought in to various computing and consumer electronics platforms. Of the 11 he's charted, Android and Apple's iOS are neck and neck viewed on the same quarter-by-quarter time scale. It's a little confusing to sort out which platforms get their own line and which just have dots but the adoption curve for Android and iOS look the steepest/most rapid.
Might also be interesting to look at other rapidly growing platforms like CDs players, DVD players etc. They don't have the identical characteristics to smartphone platforms but then neither do video games or online services.