I say go for it (for people), as long as there are sufficient restrictions in place that prohibit crap like changing eye color or whatever. If it's legitimately medical, then good. If it's not, the parents would be exceeding their authority. There was a funny article on /. about people thinking of modifying the "gay" genes in their kids so they'd be godfearing straight people. LOL!
As for the cats, I really don't see any reason not to, as long as the manipulation doesn't cause harm to the kitty.