BARCELONA—Microsoft CEO Steve Ballmer kicked off the keynote portion of Mobile World Congress (MWC) late Monday afternoon with what seemed like a most appropriate guest: Nokia CEO Stephen Elop, with whom Ballmer made the announcement about a new strategic partnership just days prior. In their view, the partnership creates what they call the “third ecosystem,” one that runs alongside iOS and Android.

Before introducing Elop, Ballmer ticked off some of the great successes in Windows Phone 7, which was unveiled here a year ago. Early feedback has been positive, with surveys showing nine out of 10 people who buy a Windows phone would recommend it to others.

For one thing, people like phones that help them complete basic tasks more easily, he said. One of those common tasks is taking pictures, and believe it or not, many phones on the market today do not make it easy to take photos and easily manage them, he said. Windows Phone 7 makes it easy to take a picture and instantly share with friends, even if the phone is locked.

Microsoft executive Joe Belfiore, who unveiled Phone 7 at last year’s MWC, returned this year to show a demo of some new features coming to Phone 7, including Internet Explorer 9, which will be coming to phones in an update release later this year.

Belfiore showed some of the differences between IE9 on a phone compared with Apple’s Safari on an iPhone 4. In an example showing animated fish, the fish barely crawled on the Safari phone screen, but they were swimming wildly fast on the IE 9 demo phone, demonstrating the difference in performance.

But back to the talk that dominated many discussions on the ground today. Just in case anyone was wondering what Microsoft’s partnership with Nokia will do for its partnerships with so many other OEMs, Ballmer seemed to try to answer that, saying it’s working with Samsung, LG, Dell, HTC and many others today to deliver differentiated phones – and in Microsoft’s mind, its partnership with Nokia should not change that. Such partners have been a critical part of the success of Windows phones, he said.

Nokia has incredible reach and expertise in areas like industrial design, camera technology and supply chain; and its maps will be used broadly in Bing. Nokia’s involvement will drive volume, he said.

As for Nokia, Elop said the deal between Microsoft and Nokia represents a natural partnership. Nokia has a strong global reach and distribution, and it can benefit from Microsoft’s relationships and standing in the U.S. market. Basically, the deal will allow Nokia to deliver products that are more competitive. That’s good news for operators, he said, because they can deliver more choice.

Ballmer expressed great optimism for the coming year for Microsoft and its partners in the wireless ecosystem. (It’s unclear where they view Research In Motion (RIM) based on their references to becoming a third ecosystem.) Lest anyone think that Microsoft is coming too late to the party, earlier in the day, Tony Mestres, vice president in the Communications Sector for Consumer Channels, expressed that same sort of optimism when asked about the competitive landscape in a world where Verizon Wireless just started selling the iPhone 4.

“We could not be more committed or excited about what we’re doing,” including with mobile operator partners. Microsoft just launched its Phone 7 phones in the United States in November, and it’s still early days. “The future … is full of incredible opportunities,” Mestres said.

Four months after release, Microsoft reports that the Windows Phone store has more than 8,000 applications and rising.