Funny you should mention that -- everybody praises my daughter for her word pronunciation, whereas a lot of her fellow students have a lisp or talk like they are babies still. My daughter tries the baby talk now and then and I just give the old furrowed brow look and she then speaks normally!
It irks me a bit when I hear that because my nephew had a really bad speech impediment that his parents refused to acknowledge. I spent a week alone with him camping when he was 10 and I was 18 and he came back being able to speak properly. I just ignored him when he lapsed into his baby talk/lisp, and he quickly got the idea, because I heard him before talk just fine if he slowed down and thought a bit.
Now, you'd never know anything was wrong.