I hope not the government though because according to the Constitution the government is NOT to establish religions. I guess individual people are free to do that or not.
I think the establishment clause refers to a state religion, as the Anglican church was the state religion of Great Britain at one time.
In any case, I think as long as Islam is considered one of the major religions in the world, we're probably obligated to recognize it as a religion, whether or not we agree with it.