It's by now well established that Muslim birthrates in many majority-Muslim countries are cascading down: total fertility rate is under replacement (2.1 children per woman) in such countries as Uzbekistan, Lebanon, Algeria, and Iran.
In addition, that same pattern also holds for Muslims living in some Western countries. Here is information on the phenomenon as it becomes available:
Denmark: From the Islam in Europe blog:
A long-time trend of fewer births among immigrants means that their birthrate is now lower than for ethnic Danes. While Danish women have 1.9 children on average, non-Western immigrant women have 1.6. … The birthrate among immigrant women has been cut by more than half since 1993, when the average woman had 3.4 children, twice as many as Danish women did.
(July 26, 2010) July 27, 2010 update: Yesterday's figures, it turns out, were in error: Western immigrant women have a 1.6 TFR and non-Western immigrant women have a 1.9 TFR. The numbers are less dramatic but the basic point remains: Muslim women in the West bear fewer children over time.