So, in the "studies that yield fairly obvious results" department, the American Academy of Pediatrics has released a report saying that children under two-years-old should avoid watching TV as much as possible.
When children in this age group watch TV, which they typically can't comprehend, it can lead to sleep problems and delayed speech, as well as lack of unstructured play, says the report.
This apparently includes passive TV watching, like, I suppose, when I watch a good hour or two of news in the morning, and my six-month-old is right there on his activity mat. Oh well.