Many of you may have heard about the study a couple of years ago refuting Disney's claim that Baby Einstein was "educational" for infants. It has been found that because watching television is such a passive activity, it really is not educational for babies, and in fact could be detrimental. For that reason, the American Academy of Pediatrics recommends no TV viewing for children under 2.
I was very good about following that recommendation with my little boy, but once he turned 2, I started feeling guilty for the amount of TV I let him watch. I was happy to find out that studies show that kids over 2 can actually benefit from watching a little bit of TV. In fact, exposure to shows such as Sesame Street, Dora, Blue's Clues, and Mister Rogers has been associated with pre-academic skills and long-term academic success. I would also add anecdotal evidence that the show, Yo Gabba Gabba, really helps kids (at least my little boy) develop some skills such as trying new things, persevering, sharing, etc.
Use your judgment. Don't sit your toddler or pre-schooler in front of the TV all day, but an hour or two a day isn't going to hurt, and might even help!