Richard Chappell asserts, just as my good friend Bob does, that it's okay to produce a person with injuries as long as that person's life is worth living. But he assumes that "the damage is moderate enough that the resulting life would still be well worth living." Notice the terms "moderate" and "well" there.
Take flipper babies, babies born with stubby arms and legs due to thalydomide poisoning. Suppose the baby has no arms or legs (or whichever horrific ailment you've supposed.) It doesn't matter how, but imagine the harm is grievous enough to make the life just barely worth living. Death by 12. Frequent pain. Minimal ability to function. But, as luck would have it, activity providing enough entertainment to register a lackluster and dreary "acceptable" on the scale of goodness of life. By no means deeply satisfying. By no means.
Now, if you knew that this was the only sort of baby you could conceive, then your choice to do it would seem to be wrong, at least by my lights. The child's looking you in the face and saying in anguish, "How could you do this to me papa?" would make sense. "But I love you and you're all I could make." "I know you love me papa." Then the head turns away. You chose to make it the case that someone has no arms or legs. You chose to make it the case that someone has a life only barely worth living. You did wrong. At least, it isn't clear that you did not, and Richard hasn't proven other wise.
The practical point of this business, to me, is sperm bank conception. It seems to me a grievous injury to choose to make it the case that someone has a father who doesn't ever love, ever see, or ever want to see him. The harm seems grievous enough to make this form of conception wrong. (Check my little series of posts over at Right Reason.)
Here my point is just that it is not in the clear that it's okay to do harm to someone as long as that he doesn't exist yet and the harm is necessary to create him. Richard's Pinnochio example doesn't do anything to refute that contention.
By the way, if you like philosophy blogs, you'll love Richard's.