I don't doubt that "plagiarism" is a persistent problem that is exacerbated in the "digital age" (see Plagiarism lines blur for students in digitial age ). I do think though that it is worth noticing that there are two somewhat different issues at stake in thinking about "plagiarism," and to highlight, particularly in educational contexts, the one that often gets less attention.
The more commonly expressed concern has to do with "intellectual property." It is of course "cheating" to pass off materials created by others as if they were one's own. To put it slightly differently, writers ought to be aware of the contributions others have made to their own creations and give credit to those on whose shoulders they are standing. There is, of course, a certain fuzziness in this mandate, and the digital age has and inevitably will continue to increase the fuzziness. We are all of us all the time "standing on the shoulders of others," and so its not at all unreasonable for students to be sometimes puzzled about the need to give to give credit for "common knowledge " and when/how to go about doing it. And there is some justice to the position that creative work always "borrows from the vortex of information to mash up " something new. That there is fuzziness does not of course diminish the inappropriateness of writing in which one deliberately or even inadvertently "passes off materials created by others as if they were one's own." And so many people are combating plagiarism by themselves using the tools of the digital age to see whether this has occurred.
My own sense is that while there is an intellectual property issue involved in "plagiarism," there is also something else that is worth paying at least as much attention to and investing at least as much energy to combat. Students, and others, frequently approach writing as something they need to do in order to complete an assignment mandated by others. In this context, the issue of intellectual property frequently, no matter how undesirably, seems (consciously or unconsciously) less important than getting the assignment done. What is being missed, of course, is the importance of writing as an individual constructive activity, as a process of taking existing material and using it to generate a distinctive perspective of one's own. The failure to do this is, in my mind, at least as significant in "plagiarism" as is offense against intellectual property. One's writing should be an idiosyncratic expression of one's own distinctive understanding, and this means, among other things, saying things in one's own words.
From this perspective, the challenge offered by "plagiarism" is not simply that of teaching people to respect the intellectual property of others but of themselves as well. And it requires not only detecting violations of intellectual property rights and punishing perpetrators for it but emphasizing the importance in writing of "an idiosyncratic expression of one's own distinctive understandings" and creating an environment that encourages that. To the extent we mandate writing from others (or ourselves) that doesn't require the creation of new, distinctive understandings, we contribute to the likelihood of plagiarism. To the extent we insist on people writing in ways that require them to construct for themselves new and distinctive understandings (cf Evolution of science education as story telling and story revising ), we make plagiarism less likely. The plagiarism problem and solution is not only in others but also in ourselves.