Android's share (of smartphone shipments) across the globe is gaining momentum according to the latest IDC numbers. By contrast there's evidence that Android's US share may have "peaked" according to analysis from Asymco's Horace Dediu.
Below are IDC's estimates showing global market share for Q2 by shipments:
Thus Android stands near 80% of global smartphone shipments, which aren't identical with sales. But it's a directional indication of actual sales.
However in the US market the story is different; Android's share is flat (per comScore):
Dediu points out that over the past six to eight months in the US the iPhone has gained more usage than Android (11M vs. 6.6M users). So it would appear that Apple's US and international fortunes have significantly diverged.
However we also have research from CIRP, which finds (via survey data) that "first time smartphone buyers" in the US (meaning those buying smartphones for the first time now) tend to be older and more price sensitive. They buy "secondary Android brands" (e.g., LG) and keep their phones longer.
Apple's strategy for more price-sensitive consumers has been the iPhone 4 and 4S, which has been reasonably successful to date. However rumors suggest a low-cost "plastic" iPhone for emerging markets and more price-conscious consumers.
When looked at in the context of overall computer operating systems (including the PC), Android will be the dominant OS by 2015 on a global basis -- far outstripping Windows. By comparision, Apple's overall OS share (iOS + Mac OS) is expected to nearly match Windows.