Ballmer yesterday gave the Mobile World Congress a detailed progress report on WP7, outlined the advances coming later this year, and together with Nokia CEO Stephen Elop explained the reasons behind the new partnership.
Both Ballmer and Elop, who joined the Microsoft chief for toward the end of his keynote, talked of the opening of a third ecosystem, which will give consumers more hardware choices and additional innovative services.
Elop said that the two companies represent a "natural partnership" that is complementary, matching Nokia's iconic devices and industry design capabilities with Microsoft's "leading operating system for the future" and backend services.
He noted that the alliance, with its global reach, gives them the opportunity to deliver a broad range of devices across the world, but most importantly it would allow them to turn out products that are more competitive.
Ballmer said that four months after launch, Marketplace has some 8,000 apps and more than 30,000 registered developers, and more than a million developers have downloaded its visual studio toolkit.
"All of the bears promise for the interesting things you'll see on the phone. We're off to a strong start, but we know we have a lot of work to do. The Windows Phone platform, like any other healthy platform, will only thrive with scale and variety," he said.