My youngest was speaking very early, but we also exposed her to ASL through Signing Time dvds which we bought for her 1st birthday. She had a verbal vocabulary of 500 words by the time she was 15 or 16 months and we stopped counting. She knew the entire ASL alphabet and maybe 100 signs around her second birthday. I don't think the signing hurt her; it only enhanced her ability to communicate. She didn't learn ASL instead of spoken English. It was learned as a second language, so she could ask for juice or milk with a word or a gesture.
I think exposure to any second language is a good thing. If a child appears to have a communication issue though, a professional should be consulted.
