In the first quarter of 2010, Apple secured a 21% market share, Android surged to 28%, and RIM's BlackBerry held strong at 36%. I don't mean to ignore BlackBerry here, but they've been dominating sales for so long, and with so little change in their fundamental business, that they're barely worth discussing from a sales perspective. It'd be like writing a post about how U2's latest album is selling well.
It was really a foregone conclusion that eventually Android would beat the iPhone in sales, for a bunch of simple reasons (including the recent study showing Android dominating mobile web use). iPhone OS is limited to one smartphone, on one network (the second-place network in terms of customers), and offers only one real model at a time (the iPhone 3G is technically still available but it's not dramatically different in design or functionality from the 3GS). Android, on the other hand, is available in several iterations from several different manufacturers in several different price points on every single major network. Want a keyboard? Go for a Motorola Cliq or Droid. Like HTC's Sense UI? Try the Droid Eris, Hero, or Incredible. Want to buy your GooglePhone direct from Google? Opt for the Nexus One. Hate your life? Try one of those Samsung phones with the custom UI that Gizmodo's Matt Buchanan compared to various unsavory bodily excretions.
So of course Android eventually came to top iPhone in market share, if not mindshare. It hearkens back to that whole Windows vs. Mac OS argument, with Android taking the Windows path: multiple manufacturers, designs, and price points leads to bigger sales, if not greater customer satisfaction.
This will all be shaken up nicely by the end of the year: Microsoft is due to launch their groundbreaking Windows Phone 7 phones and HP may re-launch Palm's WebOS with new hardware and enough money to break into the big time. But Android will probably continue to gain users (and possibly take over the world).