Worldwide smartphone unit sales will grow 57.7% year-to-year to reach 468 million in 2011, and Google’s Android mobile OS will capture 49% of the market by 2012, according to Gartner research. Open OS devices will make up 26% of unit sales this year and will exceed 1 billion by 2015, when they will account for 47% of sales worldwide.

Google and Microsoft will gain the most in terms of market share percentage and Apple’s iOS’s will hold relatively steady, according to Gartner’s forecast, while Symbian and RIM’s market share will suffer.

“By 2015, 67 percent of all open OS devices will have an average selling price of $300 or below, proving that smartphones have been finally truly democratized,” said Roberta Cozza, principal analyst at Gartner.

“As vendors delivering Android-based devices continue to fight for market share, price will decrease to further benefit consumers”, Ms. Cozza said. “Android’s position at the high end of the market will remain strong, but its greatest volume opportunity in the longer term will be in the mid- to low-cost smartphones, above all in emerging markets.”

Apple’s iOS will hold on to second place in terms of market share through 2015, despite decreasing slightly from 2012 onwards. Gartner’s analysts assume that Apple will stay with its premium pricing strategy and not chase market share.

Symbian’s market share stands to drop from 37.6% in 2010 to 19.2% this year, and then fall more dramatically, to 0.1% by 2015. RIM’s share is forecast to drop steadily from 16% in 2010 to 13.4% this year, then down to 11.1% in 2015, though Gartner’s analysts believe that RIM’s transition to the QNX OS is a smart move and will work out well.

Microsoft’s market share, by contrast, is expected to gain 1.4 percentage points this year, to 5.6%, and increase to 19.5% by 2015. That’s thanks in large part to the recently announced partnership with Nokia, which will drive the Windows Phone OS into third place.

Gartner analysts said new device types will widen ecosystems. “The growth in sales of media tablets expected in 2011 and future years will widen the ecosystems that open OS communications devices have created. This will, by and large, function more as a driver than an inhibitor for sales of open OS devices,” said Carolina Milanesi, research vice president at Gartner.

Growth in tablets, smartphones and other mobile telecomputing formats will continue to drive adoption and use of open OS devices, according to Gartner’s research. “Consumers who already own an open OS communications device will be drawn to media tablets and more often than not, to media tablets that share the same OS as their smartphone,” according to Gartner vice president of research Carolina Milanesi. “This allows consumers to be able to share the same experience across devices as well as apps, settings or game scores. At the same time, tablet users who don’t own a smartphone could be prompted to adopt one to be able to share the experience they have on their tablets.”