I recently received my copy of the exquisite Atlas of Global Christianity. It is a treasure-trove of statistics, maps, and information that is well worth the price for any scholar of religion (especially since it comes with user-friendly dvd of the contents).
The map above comes from the Atlas and depicts Muslims by province in 2010. Also depicted is the statistical center of gravity for global Muslims in 2010 with a comparison of that statistical point 100 years ago. In some ways, the slight southward shift of Muslims in the last 100 years is no surprise – Muslims in Pakistan, India, and Indonesia continue to grow (by fertility alone even). The westward shift seems at first a bit counterintuitive though. The growth of Islam in Africa and emigration of various Muslims to the West must account for this to some degree.