I was reading through journal articles for an assignment and came across one in the Journal of Infant Behaviour and Development (Possibly the most interesting journal I have discovered during my studies). It looks at the possible link between types of TV shows and delayed language development in toddlers. (Okuma, K. & Tanimura, M. (2009) A preliminary study on the relationship between characteristics of TV content and delayed speech development in young children. Infant Behaviour and Development, 32, 312-321)
And there is a link between 'infant education' shows (like In the night garden, Teletubbies, Baby Einstein) and language delay in young children. The problem is not so much the shows themselves, but responses they evoke in young children. They have few close ups of a person talking to the camera (and consequently the child) which in turn discourages children to interact with the show and the people with them who are watching the show. Children also don't associate what they hear with facial expressions and so some of the information they need to make the connection between speech and communication is getting jumbled.
Shows which have a person 'talking' to the child encourage the child to imitate the noises the person is making (beginning of talking), they will look to the person who is with them, point and 'comment' about the show. The 'infant education' shows cause a child to stare at the screen and not interact with people around them.
Also, shows which have a positive link to language delays were made up of segments (think Hi-5, Playschool, The Wiggles). The reasoning in this is that the brief pause between segments (as the camera pans, or the people gather different props or change location) allow the child to shift their attention away from the TV and once again interact with people with them.
I'd always been a bit disturbed by 'infant education' shows, I'm pleased I now have a real reason to dislike them :).