Me and a friend had this discussion once. Basically, the discovery of alien life doesn't remove the possibility of God existing. Like GirlFriday said, who is to say that God stopped at us? Or even started with us? The problem with religion is that it has always been tainted with man's vanity. A few examples: everything revolves around the earth, man created in God's image, and God being perfect. God must be perfect because he created man and only a perfect being could create something so wondrous. There is no alien life because why would God create more life somewhere else when he succeeded with us? Human kind has shown time and time again that we are hardly perfect, and that we hardly adhere to God's so called teachings. Personally, I kind of think that if there is a God, then he put life all over the place. Assuming He created everything, why create a Universe and not fill it? Even if everything comes down to physics and the Universe dies as the last star fades away, who is to say that God didn't want it that way?
So, in the end, while the discovery of ET life might have an effect on the specifics of each religion, it would hardly shatter the faith of the devout or change the overall teachings of each religion.
edit: i'm a part-time agnostic, part-time catholic.