The American Academy of Pediatrics recently updated their policy statement addressing media use in children in October 2011. "The AAP discourages media use by children younger than 2 years. Unstructured playtime is more valuable for the developing brain than any electronic media exposure. If a parent is not able to actively play with a child, that child should have solo playtime with an adult nearby. Even for infants as young as 4 months of age, solo play allows a child to think creatively, problem-solve, and accomplish tasks with minimal parent interaction. The parent can also learn something in the process of giving the child an opportunity to entertain himself or herself while remaining nearby." A television or other electronic media is not a substitute for the interaction between a parent and child.
A study published in 2007 in the Archives of Pediatrics and Adolescent Medicine found that "By 3 months of age, about 40% of children regularly watched television, DVDs, or videos. By 24 months, this proportion rose to 90%. The median age at which regular media exposure was introduced was 9 months. Among those who watched, the average viewing time per day rose from 1 hour per day for children younger than 12 months to more than 1.5 hours per day by 24 months. Parents watched with their children more than half of the time. Parents gave education, entertainment, and babysitting as major reasons for media exposure in their children younger than 2 years."
I find these statistics astounding considering that it has not been proven that watching television or engaging in any form of electronic media in children under 2 has had any educational benefit. Heavy media use has been "associated with obesity, sleep issues, aggressive behaviors, and attention issues in preschool- and school-aged children." It may also cause eye strain if a child sits too close to the television (which I know many young children do).