Alex Beam has an interesting piece in the Boston Globe on copyright issues surrounding Roy Lichtenstein's use of images from comics. An art teacher named David Barsalou has been tracking down and cataloging specific comics that were the inspiration for Lichtenstein's paintings; so far he's found about 140. "Color me naive," writes Beam, "but I never thought Lichtenstein's work was a direct copy of scenes from comic books. I assumed that he stylized certain scenes suggested by the comic vernacular of the 1950s and 1960s." He also correctly points out that Lichtenstein could have faced serious copyright problems (Beam doesn't mention it, but just think of Rogers v. Koons); he says the interesting question is why he never did. The question is in any case now moot: there's a three-year statute of limitations for copyright claims.
You can see samples of Barsalou's research at his website, Deconstructing Roy Lichtenstein.