I have a great interest in brain development, so I've read a number of books and attended seminars about various topics- so, I'm certainly not an expert, but I did retain a few interesting nuggests.
It turns out that when they've done brains scans of babies watching television, that the active part of the brain is not one that reflects curiousity and interest, which is what all of us think is happening when we see a baby riveted by the television. Instead, they are (as another respondant said) being completely overwhelmed by all of the stimulous. Not so good. That's one reason the American Society of Pediatrics recommends absolutely NO television before the age of 2.
Everyone does what they have to in order to get through Mommyhood in one piece, but I think it's good to have good information so you're making an informed decision.
http://www.msnbc.msn.com/id/5933775/
As an aside, I really get my feathers ruffled by companies like Baby Einstein who claim to have "educational" videos for babies when studies have shown that it overstimulates the brain, that there is absolutely a postitive correlation (not causation) to ADD, and that research shows that babies definitely do respond to one on one interraction and conversation as far as learning new words and concepts goes, while there is nothing to prove that watching a video of a real cat or whatever teaches them about a cat.
Okay, I'm stepping down from my soapbox. Sorry.
Anyway, all of my friends let there kids watch TV from early on, and I think they're all adorable and I don't judge their parents. Everyone has the right to make their own choice! Plus, even researchers advocating "no TV" readily share that much much more research needs to be done; they don't pretend to know everything.
Well, I better go. Days of Our Lives is on. : )
J.