IDC issued a reportedly recently, predicted that, by 2015, Windows Phone will be the world’s second most popular mobile operating system, trailing only Google’s Android. Most of us in the industry were a bit baffled by the news, of course, largely citing Windows Phone 7’s lukewarm sales figures as the basis for disbelief.Last night, perhaps not coincidentally, Windows Phone senior director Brandon Wilson wrote a post for The Windows Phone Developer Blog titled “A Year Later–The Windows Phone 7 Numbers That Matter.” According to Wilson, “When considering the various measures of progress over the last year, we tend to focus on the third party developers who have built so many amazing apps and games for the platform.”He goes on to cite the number of times that Windows Phone Developer Tools has been downloaded (1.5 million), the number of members of AppHub who are also members of the Windows Phone developer community (36,000), and the number of apps available for the platform (11,500). For that latter number, Wilson insists, “we have always been focused on quality over quantity.”But really, there’s one number that will determine Windows Phone 7’s success above all others: one billion. That’s the amount, in dollars, that Microsoft reportedly paid to ensure that Windows Phone 7 will be implemented on the next generation of Nokia handsets. Nokia, while not exactly a powerhouse in the US, is still the biggest manufacturer of smartphones in the world. If anything is going to determine Microsoft’s success in the mobile space, it’s that.