It can go a long way, methinks, as long as it's medical only. Fetuses can already undergo surgery while in the womb if they get ill or develop other kinds of problems, genetic treatment of issues is no different in my eyes.
There was a funny article on /. about people thinking of modifying the "gay" genes in their kids so they'd be God-fearing straight people.
Obviously there are going to be such suggestions, from "make my kid tall" and "give him black hair" to "alter his genes towards an atheistic leaning" or "for G_d's sake, erase that fag gene". We'll see where it leads.