I recently read the book "Nurture Shock." They present evidence that shows that babies who are shown Baby Einstein videos are actually less verbal and have lower cognitive abilities than those who are not. The explanation is that babies learn from responsiveness and interaction with people, not from sitting in front of the tv.
For toddlers, there are studies showing that kids who watch "kids" shows, from Sesame Street to Sponge Bob, actually have more emotionally aggressive (name-calling, leaving people out, that kind of stuff) behavior than even those kids who watch violent television (who, not surprisingly, are more physically aggressive). Apparently, what they think is going on is that the kids shows are meant to "teach a lesson," so a character goes around doing something "bad/wrong" the whole episode, and then learns at the end that it was bad/wrong. While adults follow this logic, young children simply see the bad behavior modeled over and over again, so are more likely to repeat it. Apparently, Sponge Bob has something like 40 instances of emotional aggression in about 30 minutes.
Furthermore, the book "the Pediatrician's Bible" says that kids who watch a lot of tv at age 2 then tend to have nightmares about some of the stuff they saw at age 3, tending to develop fears of things they didn't understand before and images they saw. No evidence is actually presented, however.
That said, I think there are programs that help kids learn. My son has watched the songs on Sesame street (we recorded a select few, like the Alphabet song) and definitely learned to sing the alphabet at age 22 months or so thanks to that, and he has since really learned all of the capital letters thanks to watching Super Why with his babysitter. (I'm a little concerned about the appropriateness of that show, since they discuss books which may have scary aspects, like the 3 pigs, for example). One show I truly love is Dora -- my son gets some Spanish, which we speak at home along with English, but all of the shows are about problem solving, and there are no negative behaviors or really scary situations.