One of the most fascinating stories of 2013 will be the battle between Microsoft (MSFT) and BlackBerry (BBRY) to establish themselves firmly as the third major mobile operating system behind Apple’s (AAPL) iOS and Google’s (GOOG) Android. According to new research from Strategy Analytics, Windows Phone has gotten itself off to a decent head start as it overlook BlackBerry’s market share in the United States for the first time ever in the fourth quarter of 2012. Of course, we need to add a pretty big caveat here: Microsoft had actually released Windows Phone 8 last quarter while BlackBerry was still working on getting its BlackBerry 10 software ready for release just last month. The real test for the two companies will likely come after the current quarter, when both BlackBerry and Windows Phone will have their flagship devices out on the market. If BlackBerry Z10 sales are as strong as early indications have shown, then Microsoft’s lead over BlackBerry could be very short-lived.