The source code for the ten-year old Symbian platform will be completely open source and available for free starting Thursday. The transition from proprietary code to open source is the largest in software history, claims the Symbian Foundation.

“The dominant operating system provider out there is Symbian,” says Lee Williams, executive director of the Symbian Foundation, “and now we are offering developers the ability to do so much more.”

Symbian, which powers most of Nokia’s phones, has been shipped in more than 330 million devices worldwide. But in the last few years, Symbian has seen more than its fair share of changes. In 2008, Nokia, one of Symbian’s largest customers, acquired a major share in the company. Nokia then created the Symbian Foundation to distribute the platform as an open source project, and began the process of opening up the source code that year.

Meanwhile, the operating system has seen new rivals crop up. Google’s Android, which is based on a Linux kernel, has become a favorite among handset makers such as Motorola and HTC. And it’s based on an open source foundation too.

Symbian’s move to open source has been completed four months ahead of schedule and it offers mobile developers new ways to innovate, says Williams. Any individual or organization can now take, use and modify the Symbian code for any device, from mobile phone to a tablet.

Similar as it may sound to Android’s promise, there are major differences, says Williams.

“About a third of the Android code base is open and nothing more,” says Williams. “And what is open is a collection of middleware. Everything else is closed or proprietary.”

Symbian is also ahead of Android in that it will publish its platform roadmap and planned features up to 2011, he says. And anyone can influence that roadmap or contribute to new features.

“Open source is also about open governance,” says Williams. “It’s about letting someone other than one control point guide the feature set and the asset base.”

But will that be enough for Symbian to steal away customers lured by a snazzier and younger rival?

Who will continue to rule the cell phone roost? Symbian, according to the research firm.

By 2013, IDC forecasts that worldwide shipments of smartphones will surpass 390 million units, growing at a compound annual growth rate of 20.9 percent.

“Mobile operating systems have become the key ingredient in the highly competitive mobile device market. Although the overall look and feel of the device will still play an important role in the buying process, the wrong choice of operating system coupled with an awkward user interface can mean the difference between success and failure,” said Stephen D. Drake, vice president, Mobility and Telecom, in prepared remarks.

Key takeaways from the report:

• Symbian will remain the No. 1 mobile OS worldwide, thanks to Nokia’s strength outside the U.S.
• Android will experience the fastest growth of any mobile operating system: from 690,000 units in 2008 to 68.0 million units by 2013 (that’s 150.4 percent compound annual growth).
• Linux and webOS handsets “will struggle.” Android will suck most of the life out of other Linux platforms and Palm’s webOS, despite steady growth, will be limited by restrained deployment and availability of devices across multiple carriers.

No mention of BlackBerry overlords RIM in the announcement, but judging by Android’s growth, that’s not a good sign for them, either.

Either way, there’s no way development market can sustain so many different platforms. And with Symbian (and Nokia) resting on its innovation laurels, it’s no surprise Android stands to benefit.

The IDC report is called “Worldwide Mobile OS 2009-2013 Forecast and Analysis.”