The Pew Forum has this interesting report on Muslims in Europe. It is chock-full of data and analysis, and should be read in conjunction with this article from the Weekly Standard.

Taken together, they provide much more material to digest as we think about how robust the "cultural" basis of a pluralistic society has to be. Dutch multiculturalism, for example, used to be based on "shared and nonnegotiable understandings of three things: language, history, and law." Radical Islam rejects all three. And in Rotterdam, for example, Muslim immigrants constitute nearly half the population and more than 60% of the school-age popupulation. My dad’s old neighborhood, in Rijswijk, a suburb of the Hague, has become a Muslim enclave, essentially foreign to someone who lived there for 20 years and visited for another 20.