The mobile software age is here.

Symbian co-founder Nokia announced Monday night that it is buying the 52 percent of the software maker that it doesn’t already own and releasing its mobile operating system under an open source license.

With that move, Symbian joins two other major platforms – the Google-backed Android operating system and Apple's OS X iPhone – that give programmers tools for creating and deploying software for smartphones.

The Symbian OS dominates the world market, with about 60 percent of the installed base among smartphones. According to Nokia, more than 200 million phones currently in use worldwide are running Symbian software. But Symbian trails in the United States, where Research in Motion, Palm, Windows Mobile – and now the iPhone – are the major players.

Nokia uses Symbian software across its range of mobile devices, primarily with the extremely popular S60 interface. Other handset companies also use some variety of the Symbian operating system, including Sony Ericsson, Motorola and NTT DoCoMo.

"Nokia could, if they found inside the corporation the resolve to do so, come out with the definitive open platform," said Bruce Perens, an open source advocate and CEO of Kiloboot. "They would have a platform of the type we haven't seen since the original Palm. When that was dominant, there were 16,000 applications available to install. The question is, can they find the corporate resolve?"

The prospect of thousands of mobile apps – instead of the few dozen typically available through most wireless carriers – is something new in the wireless world. And the 6 million iPhones sold to date show that mobile users like having open, unfettered access to web applications and online content.

In short, what matters to handsets now is not so much features, graphics chips and innovative interfaces – though those do help. What's critical is an easy-to-use development platform that enables programmers to create a wide range of software quickly and easily, so that they can give consumers the content and the software they demand.

Android (whose first handsets are expected later this year) is clearly aimed at that goal. And while it's not open source, Apple has built a complete developer ecosystem around the iPhone, including everything from development tools to a store (which will open next month) for selling finished applications.

That's a significant shift from just a year ago, when programming tools for handsets were specialized and difficult to use, and carriers and handset manufacturers alike kept a tight rein on mobile application deployment.

To support the new open source project, Nokia is establishing the Symbian Foundation, a collective of hardware and software companies that have pledged to donate code and resources to Symbian's development. Phone makers Motorola and Sony Ericsson are on board, contributing software from their UIQ project, a touchscreen interface for Symbian. Japanese carrier NTT DoCoMo has pledged support and is contributing its Symbian interface, MOAP(S). Other supporters include AT&T, Samsung and Texas Instruments.

"Establishing the foundation is one of the biggest contributions to an open community ever made," said Olli-Pekka Kallasvuo, CEO of Nokia, somewhat hyperbolically. But it is true that Nokia has, at one stroke, created an enormous open-source ecosystem, thanks to the huge number of Symbian phones already in use.

Nokia's move is a defensive one, of course. The Symbian Foundation plan is strikingly similar to Google's plan with the Open Handset Alliance, a collective of industry players who have come together to build and nurture the Android open source mobile operating system. On the carrier side, Google has NTT DoCoMo, Sprint Nextel and T-Mobile on board. On the hardware side, HTC, LG, Motorola and Samsung have signed on to support Android.

Nokia says it is even taking a Google-like approach to rolling out the open source code. It will release components of its code under an open source license at first, with the full OS to follow "over the next two years." Right now, Nokia says, it intends to release Symbian under the Eclipse Public License (EPL) 1.0.

But not everyone is convinced that open source operating systems are the way to go.

"With the success of Apple's and RIM's models, we would have thought traditional handset vendors would develop and maintain similar proprietary OS models," said Tavis McCourt, a Morgan Keegan analyst. "We view this move as a long-term positive for the smartphone vendors that own their own OS (RIM, Apple and, soon, Palm)."

And it's still too soon to tell which mobile platform will win out. Symbian has the advantage of a large installed base; Android will benefit from the pure innovation seen when developers take a "sky's the limit" approach to building a new OS. And Apple provides a complete, turnkey approach to software sales via its iTunes App Store, which may appeal to consumers.

One thing's for sure: The floodgates are opening, and the coming year will see an explosion of mobile software for a wide range of smartphones.

Additional reporting by Betsy Schiffman.