The title pretty much says it all - do actors, directors, writers, etc. use the internet to get immediate, relatively un-biased feedback on their projects? If not, why wouldn’t they?
(Do you figure any famous people read the Dope and hang on our every word about them?)
There’s indication Jon Stewart does, as he’s commented about negative reactions to the set redecoration on The Daily Show and his apparent wussiness in interviewing Rick Santorum. His comments came very rapidly, suggesting he was getting near-instant feedback.
I think it’s more likely that someone looks it over and summarizes things for Stewart, but you never know.
The actors and writers of Arrested Development apparently check out Television Without Pity all the time. There was a link to a long TWoP interview with Will Arnett (G.O.B.) around here sometime back, and he talked about how they slipped a reference to the site into an episode and how go there and look for reactions.
It’s well-known that J. Michael Straczynski hung around Babylon 5 message boards. So did Tim Minnear, David Fury and other writers from Buffy the Vampire Slayer. The catch is that they have to steer clear of anybody posting story ideas, because of copyright problems and such. But they definately read and listen. Hell,** Lost** this season even had a nod to the Internet fans in the character of Arzt, who vocalized any number of fan questions/observations/complaints! 
Entertainment gatekeepers on the highest levels – network TV executives and Hollywood studio brass – will probably never use the net to gauge public opinion, because they really don’t give a wet shit what the public thinks of their product, only about what $ it rakes in.
Some Simpson writers do. Trey Parker says he loves internet stuff that slams them as provides story ideas or jokes sometimes.
Well, one anecdotal case is nothing more than that, but an acquintance of mine is a fairly highly placed executive for programming for two major cable networks – ones you’ll find in pretty much every basic cable package. I can’t say for sure whether or not he trolls the net for feedback, but he’s extremely net-active, and it seems unlikely to me that he would miss anything significant being said about his networks. He and his former boss, I know, used to personally review and respond to e-mails from viewers sent to an e-mail alias that was widely published and broadcast – that was several years ago, and I wouldn’t be surprised if the volume of e-mails would simply be too much to deal with now, but at least in his case I know that he does have an interest in what people think.