Pew has a new report on projecting the Muslim population around the world for 2030. You can look at separate reports by region and there is a lot of interesting information. If you look at the data for the United States, the prediction is that there will be 6.2 million Muslims by 2030. This is still a relatively small percentage compared to the total population though this would be a 140% increase. The numbers for Europe are quite different: the projection is France, Belgium, and Russia will be more than 10% Muslim.
Lots of good data here on everything from fertility rates to migration to age breakdowns.