Nielsen ratings are audience measurement systems developed by Nielsen Media Research. NMR originally focused on radio and TV popularity but has recently turned its hand towards smartphone sales. Take a detailed look at their graph presented to the right hand side.
As of July2010 Android out-sells RIM Backberry OS and Apple iPhone OS and shows substantial growth since January 2010, when it only accounted for 14% of new sales. It now stands tall at 32% of sales, while RIM and Apple offer 25% and 26% respectively (there is some weird labelling on the graph for the final month if you ask me).
If you are interested in current smartphone users, as opposed to new acquisitions, then head on over to the source. It shows that RIM still have the lead but are falling, while the iPhone remains steady.