Interestingly, most of this growth seems to have come at the expense of Blackberry, and to a lesser extent Microsoft. Apple's market share has decreased slightly, but remained mostly stable. Smartphone adoption also increased more than 10% last year, so I suspect much of Android's success is coming from new smartphone owners that are buying the newest, shiniest model available, rather than abandoning their old phones.
What does this mean for marketers?
The field is now more tightly packed. Early in 2010, Blackberry had nearly 50% of the smartphone market, with Apple in second place at less than 30%. Now, Apple, Blackberry, and Android are within about 5% of each other. This means it is no longer as viable a strategy to design a system-specific app unless you plan to make versions for each operating system. Even as the new front-runner, an Android-only app will ignore almost 70% of the market. The only exception to this is if you have done enough market research to know that your clients have a strong preference; this is, however, not likely to be the case.
Instead, look at creating web-based apps. Make these more than dedicated mobile sites by including functionality that clients will need while out of their office, and make it easier to treat it as a "dedicated app" by including instructions to install a shortcut on your smartphone's home screen.
Part of Android's success, in my opinion, comes from the same strategy that helped Microsoft beat Apple in the world of PCs: their operating system is not tied to a specific hardware. This means people can shop around more easily for lower price, and feature sets that better meet their needs. But it also means there is less predictability about what features any given smartphone will offer. Design your web apps and mobile web pages around the most common features (touch screen, some form of keyboard, cameras, etc.), and do not be overly reliant on advanced features that may not be standard (gyroscopes, GPS, video).
It will be interesting to see how the continued emergence of tablets into mobile computing will effect these rankings. Will iPad's popularity and early technological leadership help Apple retake the lead, or will Android-based tablets win out for the same reasons their phones did?