The Windows Phone Developer blog has published an info-graphic that offers a summary of key metrics from the Marketplace for 2011. 56% of Marketplace users have access to operator billing, the average user has downloaded 48 apps, 50,000 apps have been published, and there are 80,000 registered Windows Phone developer (around 13,000 have published an app)
The Marketplace categories driving the most downloads for paid apps are games (64%, likely skewed by the Xbox Live titles), tools & productivity (8%), entertainment (8%) and travel (4%); for free apps it is games (31%), tools and productivity (18%), entertainment (13%) and music and video (8%).
There's no metric for the total number of downloads from the Windows Phone Marketplace, but, given the downloads per use figure, the number is probably somewhere in the few hundreds of millions area.
