I know I'm going to tick just about EVERYONE in here off but...
If you want to tell me that you're afraid someone will recreate Hitler? Fine. That's a valid fear. If you're worried about retardation or slowing evolution or cloning degredation? Fine. Those are valid as well. However, I refuse to accept God, Church, or religion as excuses for holding back scientific advancement. Bottom line: not everyone on this earth is a) Christian or b) even believes in God in the first place.
Obviously, there's a line in the sand - I guess the REAL question is; where is that line? Would it be wise to try to bring extinct species of plants and animals back to life? Would we learn lessons that would make our world healthier and happier? What if they could find a "cure" for aging? What if they could find a cure for cancer or aids or the common cold? What if your mother had one of these diseases? What if YOU had one of these diseases?
This is a new frontier. As with any frontier, there are bound to be dangers and wonderous discoveries. My hope is that the good outweighs the bad.