My son started watching Sesame Street pretty early. It was the only show he watched FOREVER. I felt weird about it, because what I read said no TV until age 3 was the best. But, he really fell in love with Elmo, and for the longest time, we'd just ffwd (Tivo/DVR) it to the parts with Elmo. Then, he started from the beginning. He learned SO much, ABC's, numbers, songs, sign language, etc. He doesn't even usually sit still when it's on (unless he isn't feeling well), but stays active playing around, running around, etc.
I am a little worried about him watching "too much" TV, but I've kept his shows really, really limited. We've added Teletubbies and Mr. Rogers...also some Harold and the Purple Crayon and Curious George. But, they're saved on our DVR, so we can turn them on and off when we choose. (That's a really nice thing, the DVR, for kids, and moms!) Anyway, those are the only shows he really watches, unless HBO family has a show about kids playing music or something. But, it's usually just a "transition" thing at our house...he has "Elmo night night", which is watching a little Sesame Street while rocking and nursing. Or he watches it a little after waking from a nap.
I would say, don't worry about it too much....just keep your ideas of how much, and when, really defined in your own mind, and stick to that.....of course, it's always good to be a little flexible on it, every once in a while.
I guess my point (long-winded, huh??) is that I was concerned about it, too, but I don't think his TV watching has harmed my son. Sometimes he asks, and I say no. But really, it's been a positive experience, especially with Sesame Street. I can't say enough about how much he's learned!! And he is so enthusiastic about many of the things he's learned watching it. We talk about it, and play, and use the alphabet, numbers and stuff, much, much more on our own (with the TV off) than he spends time watching it on TV.