PolitiFact
PolitiFact, the fact-checking operation run by Poynter's St. Petersburg Times, looked into  Jon Stewart's statement to Chris Wallace, “Who has the most consistently misinformed media viewers? Fox. Fox viewers. Consistently. Every poll.” Louis Jacobson writes that three Pew studies "superficially rank" Fox News viewers low in knowledge of current events. But people who rely on other general-interest media also rank low, and viewers of some Fox News programs rank high (along with those who watch Stewart's show). Two other surveys show mixed support for Stewart's statement.
Jacobson writes:

"The way Stewart phrased the comment, it’s not enough to show a sliver of evidence that Fox News’ audience is ill-informed. The evidence needs to support the view that the data shows they are 'consistently' misinformed -- a term he used not once but three times. It’s simply not true that 'every poll' shows that result. So we rate his claim False."

Reaction: Readers say PolitiFact failed to distinguish between "misinformed" and "ill-informed."
Earlier: Stewart's best lines to Wallace on "Fox News Sunday"