I think it depends on what they are watching. I allowed my son (who is now 29) to watch educational shows when he was a toddler. I used them as a learning tool, not a baby sitter.
At the age of 6, he actually had a discussion with a professor on the movie ratings system, and it blew the guys mind away.
I don't care what the studies show. It's how you use the TV to benefit the child's mind. We would play games from the show, or use the show in various ways after it was over. He developed one hell of an imagination from all the games we played. TV, when used properly can help a child learn manners, colors, shapes, and sizes.