This is GOING TO HAPPEN no matter what neither you, nor I, nor President Bush, nor President Fox, nor Fidel Castro do to stop it.
It can turn into a very good thing, IMHO. Living in South America (in Chile) I can see first hand that these people have a great deal to contribute to the U.S. and will in the end continue the American dream, not end it. They are in general MUCH more religious and of higher moral character than many or even most Americans.
There is a selection effect w/ respect to the immigrants (or at least there used to be). Only the ones willing to make the effort to leave their own country and make a go of it in the U.S. would show up, and these were very hardworking and resourceful.
That has changed a lot now thanks to the People's Republic of Kalifornia welfare rules. Sigh...
Anyway, I have NO DOUBT that Islam will end the European dream (a self-styled 'culturally superior', and in no small way racist Social Democrat cradle to grave paradise), or co-opt it into a Islamic Socialist Empire extending from Scotland to Malaysia...