Plagiarism today is heavily invested with morality surrounding intellectual honesty. That is laudable. But truly distinguishing plagiarism is a matter of intent. Did I mean to copy, was it accidental (a trick of memory), was it polygenesis. Up until now, intent has largely been determined at a functional level (how many lines show up in another source) and the burden of proof weighs heavily against a student.
Now lets add, Google, Amazon, Wikipedia, and TurnItIn.com into the equation. Lets go a few years down the road and assume Google has finished scanning the Harvard library, Amazon has expanded there available chapter views, and both Wikipedia and TurnItIn have tripled in size (say 7 years). How many students, writing term papers on old chestnuts like Jane Eyre are going to be able to write a full paper without having several sentences with an 80-100% match with some other source.
So, if schools continue to use papers as primary assessment tools; (I had college classes where papers were 100% of my grade, and 60% or so in high school) and you can fail a paper or be expelled for plagiarism of as little as 2-3 lines; there is a visible point where this system will break down. Schools need to develop more comprehensive systems that include the paper, oral reporting, in class writing, and more in proper ratios. If they do not many innocent students will have their academic careers derailed and schools and teachers will waste millions of dollars on tools (like TurnItIn) that are only assuring they will see an "increase" in plagiarism because there is only one way to write "George Washington was the first President of the United States".
Finally, schools view this so narrowly as a morality issue, that most do not help students to develop the "plagiarizing" skills I use at work on a daily bias. Worse students frequently leave schools thinking that the rules of plagiarism are law, when the real laws are copyright and far more nuanced. It would be nice to see schools develop more models where they synthesize (through a combination of copying and original thought) multiple documents into a complete, single-voiced whole.