Things change quickly in the U.S. smartphone market.

According to The Nielsen Company's monthly surveys of U.S. mobile consumers from July-September 2010, consumers planning on getting a new smartphone had a very clear preference: A third (33%) wanted an Apple iPhone. Slightly more than a quarter (26%) said they desired a device with the Google Android operating system (OS). And 13 percent said they wanted a RIM Blackberry.

But consumer preferences can be fickle. Those same surveys for January 2011 – March 2011 show just how much things have changed: According to the latest figures, 31 percent of consumers who plan to get a new smartphone indicated Android was now their preferred OS. Apple’s iOS has slipped slightly in popularity to 30 percent and RIM Blackberry is down to 11 percent. Almost 20 percent of consumers are unsure of what to choose next.

Those dynamics are already translating into sales. Half of those surveyed in March 2011 who indicated they had purchased a smartphone in the past six months said they had chosen an Android device. A quarter of recent acquirers said they bought an iPhone and 15 percent said they had picked a Blackberry phone.

Which brings us to the installed base of smartphone consumers: As of March 2011, 37 percent of mobile consumers who owned a smartphone had a device with an Android OS. Apple’s iOS, claimed by 27 percent of consumers, is now outpacing Blackberry, which has 22 percent of the market.