Mobile Stats

mobiThinking is a great site for free information on the mobile technology on the global scale. It includes practical guides to mobile agencies, ad networks, top mobile markets, interviews and analysis, showcase sites and case studies, industry events and awards, a comprehensive list of links to mobile resources and a compendium of mobile statistics, which can be found under the "Global Marketing Tools" tab, Global mobile statistics 2011

AdMob Metrics is another good source for tracking this data, although their latest reports are March and May 2010.

NielsenWire has some data on the State of Mobile Apps, as well as other blog posts in their "online + global" section.

Of course, there's the ever reliable Pew Internet & American Life Project's Mobile Access 2010 for a thorough analysis of United States data.