Today's Peanuts reprint, from March 8, 1962 (they're now actually showing the original publication date at the comics.com website, by the way), is one of my favorite examples of a self-aware, self-referential strip. I'm not sure if it was one of the strips that was not reprinted before The Complete Peanuts; I think I saw it in a Peanuts book once, but I can't be certain. (Click to enlarge)
This is one of the very few self-referential strips Peanuts ever did. One of the other ones is also from 1962 (and which I'm almost certain I never saw before The Complete Peanuts), where Charlie Brown turns off the TV in disgust and wonders "what would happen if comic strips showed nothing but reruns all summer?" And an early strip has a very rare open breach of the fourth wall, where Schroeder says "sometimes I think I should put in a transfer to a new comic strip!"
Of course Bill Watterson was presumably tipping his hat to the strip reprinted above when he did virtually the same strip for Calvin and Hobbes: four panels of Calvin and Hobbes from the neck up, never changing expression or doing anything, as Calvin explains that "Grandpa says the comics were a lot better years ago when newspapers printed them bigger. He says comics now are just a bunch of Xeroxed talking heads because there's no space to tell a decent story or to show any action. He thinks people should write to their newspaper and complain."
Though the two strips are a little different in their potential targets. Schulz seems like he's making a bit of fun of his own strip as well as its critics, since the scene is only a slight exaggeration of what happens in a typical Peanuts strip (two characters leaning on a wall and talking for four panels). Watterson is the "grandpa" that Calvin talks about, the guy who crusades against small-print comics and talking heads; he's making fun of the kind of strip he hates and also making a bit of fun of himself in the punchline. ("Your grandpa takes the funnies pretty seriously." "Yeah, mom's looking into nursing homes.")
Walt Kelly in Pogo did a lot more meta-strips than either Schulz or Watterson, or indeed just about anybody else; he would do deadpan jokes about the art of comics or newspapers that had threatened to drop the strip, and he'd also have the characters break the fourth wall and admit they were in a comic strip. (One of my favorites was when Pogo started to say something that was mildly suggestive and Albert stopped him: "It's Sunday!" A reference to Kelly's belief that Sunday strips should be family-friendly because they had a younger readership.)