Horace Dediu, Asymco:
The iOS platform as a whole reached 316 million cumulative units at the end of last year. The iOS platform overtook the OS X platform in under four years and more iOS devices were sold in 2011 (156 million) than all the Macs ever sold (122 million).
Dediu gives another interesting statistic via a quote from Tim Cook. It took Apple 22 years to sell 55 million Macs. The iPad reached that mark in two years. Astounding.