Microsoft greased its marketing machine, hiring entertainers such as Katy Perry and Maroon 5 to promote its long-awaited Windows Phone 7 in November. Like competitor Apple, Microsoft chose AT&T -- as well as T-Mobile -- as its primary sales partner in the U.S. Initial demand appeared high, with devices selling out within the first day. Its newest telephone offering will propel Microsoft's share of the worldwide mobile OS market to 5.2% in 2011 from 4.7% in 2010, Gartner predicts. However, by 2014, the researcher expects Microsoft's share to fall to 3.9% as competitors Apple and Google gain share with their respective OSes.