As Compo first said, if I've taken my son out for fresh air and play and then he watches some tv I think it's fine. All about balance, and different ways or learning, relaxing and giving us a break too!
In fact I think it's good for kids to watch age appropriate television, especially if they are sat down for 1-2 progs, and not just left with the channel running on to other progs. My son learns so much from his favourites, Kipper, Maisy, Charlie and Lola, Tigger and Pooh, Fireman Sam, Mister Tumble. Mostly he's learning words, about helping, sharing, cause and effect etc.
I hate him watching anything too American, especially anything where they say "good jaaaaab!!" instead of well done ...
The problem I find is that having invested in Sky + even at 2.5 my son knows we can put on a programme on demand for him. Good as that is to save the age appropriate stuff, my husband and I can't lie and say it's not on, plus think that too much choice can be a prob.
Especially as when we were a bit older than him we would watch a nature programme or an old film or musical or whatever because it was the only thing on in "our day" (remeber the Saturday Matinee after the wrestling??).
But it was nice; nowadays kids have got so much choice on TV, and DS's and Wii's etc that they don't have to broaden their horizons past those interests. Hence when my imminent baby girl is older I shall be getting out the Sound of Music and other musicals to watch together.