http://islam.about.com/library/weekly/aa060401a.htm

It wasn't until the Ottoman Empire that the crescent moon and star became affiliated with the Muslim world. When the Turks conquered Constantinople (Istanbul) in 1453, they adopted the city's existing flag and symbol. Legend holds that the founder of the Ottoman Empire, Osman, had a dream in which the crescent moon stretched from one end of the earth to the other. Taking this as a good omen, he chose to keep the crescent and make it the symbol of his dynasty. There is speculation that the five points on the star represent the five pillars of Islam, but this is pure conjecture. The five points were not standard on the Ottoman flags, and as you will see on the following page, it is still not standard on flags used in the Muslim world today.