Turns out not all TV is necessarily always bad for all children: check out this article on gifted children and the benefits of TV: http://community.seattletimes.nwsource.com/archive/?date=19930521&slug=1702386
According to the study's authors gifted children are more actively engaged with TV and actually do learn from TV (if it is the right program of course).
My son is 4 and he loves "The Magic Schoolbus", "Sid the Science Kid" and other shows like that. He has learned a lot from these shows and as soon as he has a new interest I follow up on it with books from the library and hands-on activities such as experiments. He learned about molecules from the Magic Schoolbus and we've been studying chemistry ever since. I think it's all about the balance ...