Over at IHE, Jeanne Zaino asks if "[i]t may be time to re-think our outright rejection of appropriation and at least begin a broader discussion about (a) whether there are real differences between plagiarism and appropriation? (b) if so, what are they? (c) under what circumstances may the later be used properly? and most importantly (d) how can we begin to begin to address these issues pedagogically?" At this point, there is actually a considerable body of work about a-d, thanks to appropriation's close relationship to adaptation--itself rather extensively theorized by now (see, e.g., Linda Hutcheon). For most instructors, the sticking point in letting appropriation and plagiarism slide into one another will derive from the assumed self-reflexivity of the former term. Besides "affect[ing] a more decisive journey away from the informing source into a wholly new cultural product and domain," as Julie Sanders argues, appropriations may "highlight troubling gaps, absences, and silences within the canonical texts to which they refer" (26, 98). Appropriations, that is, call attention to how they radically rewrite their source materials, as when Jane Smiley makes King Lear's "villainous" daughters into the protagonists of A Thousand Acres. Young Jean Lee takes this process a step further in her Lear, which not only jettisons Lear himself, but also intercuts her own characters' reflections on loss and mortality with excerpts from King Lear and material lifted directly from Sesame Street's famous episode about the death of Mr. Hooper--an act that asks us to think about, among other things, our understanding of Shakespeare's "originality."
Appropriation, then, normally does not say "all this material is mine," whereas plagiarism asserts its originality in the act of theft. When Scott G. F. Bailey rewrites Hamlet in his clever The Astrologer, he expects the reader to notice both the parallels (hey, that Vibeke character sounds awfully like Ophelia!) and the subversions (the Hamlet-figure is, shall we say, somewhat less admirable than his model). By contrast, the student who hands in a paper cobbled together from multiple websites may indeed be reworking arguments, if only by accident, but the paper's success demands that the instructor not notice and/or not know how to use Google. Moreover, the plagiarist, when quizzed, may often be unable to explain "their" argument, whereas the appropriator self-consciously puts his or her work into conversation with another. In appropriation, the argument emerges from one creator's engagement with another; in plagiarism, the "creator" does an end-run around the engagement, and hopes that the audience will do likewise.
Now, does appropriation work in, say, a literary-critical context? Both Susan Gilbert and Sandra Gubar's The Madwoman in the Attic and Henry Louis Gates, Jr.'s The Signifying Monkey are famous and much-discussed1 acts of theoretical appropriation, making Harold Bloom go where he had never gone before (and significantly revising his work in the process). Granted, Gilbert, Gubar, and Gates appropriate using the forms of academic scholarship--the footnotes, the explicit discussions of the source material, and so forth. But they're still appropriating Bloom, not simply applying him. In theory, students could be asked to do an equivalent exercise, although it requires a lot more knowledge and theoretical savvy to carry it off than they likely possess. But nobody would call such appropriation "plagiarism."
1 For the former, see Holly A. Laird, Women Coauthors (Urbana: U of Ilinois Press, 2000), 40-42; and the latter, Kenneth Warren, rev. of The Signifying Monkey, Modern Philology 88.2 (Nov. 1990): 225.