Agree with Shel, this from Faire Jour is right on target
We started ASL with our child even before we got the diagnosis of profound deafness -- our state provides family sign lessons in the home (on weekends!), thinking that whether it was a cognitive disorder, deafness, an ear infection or a blockage with hearing loss lasting a couple of months, we didn't want to lose a moment without developing language during that critical time. And if things had been different and suddenly ears were working, she was speaking, and all that, it would still have been worth all the effort: she'd start off bilingual! We're convinced that having ASL as a first language has made it so easy for Li to pick up spoken English..