The unveiling of the mobile UI in February at created a lot of excitement and argument. But the latest Comscore report shows that the existing Windows Mobile market share continues its dramatic . 

Of the 42.7 million U.S. smartphone users, the percentage using Windows Mobile devices dropped 4 points, from 19.7% in October 2009 to 15.7% in January 2010. Research in Motion remains the leading smartphone operating system provider in the United States with 43% for its BlackBerry software, growing 1.7% during that period; Apple holds the No. 2 spot, at 25.1% for the iPhone, but a much slower growth rate for that period, just 0.3%.

Given those numbers, it's imperative that Microsoft demonstrate it can recast its mobile development environment as completely as it has the user interface in Windows Phone 7. Based on the MIX Windows Phone sessions, here's where the priorities and challenges lie. (A full list of the sessions can be found )

1. The Windows Phone 7 platform