Professors use essay-comparing software already. It's not new, but it does a pretty effective job. It's pretty straightforward to look for matching fragments, even tiny fragments like Markov chains or statistically improbable word pairs. However, if you compare some feeds, say, Reuters to Associated Press on the same day, even those criteria would fall apart.

Separately, you might need to refine your personal definition of plagiarism. I'd say that the two examples below show that a fairly mechanical paraphrasing is going on, but it's not clear that it would rise to the definition of plagiarism. Also, how do you allow for proper quotations? 

Macbeth is presented as a  mature    man of definitely established cha
+racter.
Macbeth is shown     as an empowered man of well-      established cha
+racter.

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In reply to Re: Brainstorming session: detecting plagiarism by halley
in thread Brainstorming session: detecting plagiarism by Ovid

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