It’s a milestone in the life of any OS: the day you reach that magical 10,000 app number. Windows Phone 7 is the latest kindred soul to achieve the feat, accomplishing the task in just over four and a half months — that’s faster than both the Android Marketplace and iTunes App Store. Microsoft’s been adding around 1,000 apps a week since it hit 5k right before the New Year, and as of late that rate’s been picking up.