My DS (now 8) watched loads of telly as a pre schooler. Admittedly, it was always CBeebies and Nick Jr (so the content was always of the type which was either educational or sending some sort of moral message) and I felt it did him good. He did loads of other stuff too in the average day, went to pre school, and did things with me too; arts and crafts, swimming, Gym Joeys, had play dates etc but the telly was always on when we were at home even if he was playing or doing something else in the same room.
He could write confidently and read ably by the time he started school (and he was a yougster too with a late August birthday).
He is bright and extremely sociable with an enquiring mind and still watches loads of telly, although now he seems to have moved away from most kids telly apart from Horrid Henry and seems to prefer quiz programmes which he watches on the Challenge satellite channel and rolling 24 hour Sky Sports News! He loves answering general knowledge questions and retains loads of it too which is great and things he learns can often feed his interest in those subjects too.
I wouldn't have been so happy to let him sit and watch endless Pokemon, Power Rangers or Disney DVDs though but he's well adjusted bright and is not withdrawn so I think the amount of telly he watched when he was really little did him no harm at all - on the contrary I feel it totally added to his pre school development.
However, using the telly as a babysitter for long periods (particularly if they're watching mindless crap) and using it as an excuse to not interact with your child is not a good thing in my view.
DH and I are telly addicts too and it's never done us any harm
Just my thoughts and expereinces...but everyone's different.