My kids have been exposed to signs, English, and Chinese since birth. My daughter learned about 2 dozen signs before her speaking ability made signing unnecessary. We only use TV as a last resort to keep the children entertained so we can get things done. My son (almost 1 year) currently uses a couple signs and several spoken words. Some words he speaks and uses the sign for simultaneously.
There's no way to know whether or not the exposure to so many different forms of communication caused a delay for my daughter, but it's hard to imagine that it did. She was using over 100 English words by 18 months, and has an enormous vocabulary now (2.75).
I actually think that if anything, signing had the opposite effect, by showing the value of communication before children are ready to speak intelligibly. Exposure to multiple spoken languages on the other hand... I can see how that would cause a delay, but I still think the advantages outweigh the disadvantages.
As far as TV, I remember reading a study that purported that children who watched educational shows like Dora and Blues Clues had bigger vocabularies at a young age than kids who didn't. By contrast, children who watched non-educational TV had smaller vocabularies.
I personally know a child who is almost identical in age to my daughter who learned from the TV many of the things that my daughter learned from interacting with me and my wife and playing with educational toys (alphabet, colors, letter identification, even foreign language exposure). Are these children learning to learn in different ways, and is one ultimately a better learning strategy? I don't know. I know the AAP recommends no TV time for children under 2, but I don't know how they came to that conclusion and if it is based on the latest research.
It would seem that a child who spends more time with an adult than a TV would have a greater opportunity to ask questions, but questions usually don't come until well after 13 months. In the early stages children learn through observation. If the TV is offering more educational content than what can otherwise be provided, then it seems to me it's a positive thing. But that's coming from someone who uses TV as a last resort.
