From Spirit magazine:

Preschoolers learned 7.5 more letters while watching television.

So all work and no play really does make Jack a dull boy. Researchers at the Education Development Center and SRI International tested how digital technologies affect preschoolers' learning capacities. For 10 weeks, students watched TV shows like Sesame Street and played video games created by the programs' producers. In the end, the media-enriched kids could better identify letters, knew more letter sounds, and better understood story concepts than their peers. Some experts think introducing media early hinders social interaction. But Shelley Pasnik, director of the EDC's Center for Children and Technology and the study's co-author, says it's all in the method: "People worry that kids will be left alone in front of a screen. But we used the media to engage students, pausing to ask questions and encourage kids to write letters and make the sounds."

Spirit Magazine