As a published author…
This past weekend saw an interesting (and lengthy) discussion on slashdot about Turnitin.com and a group of high school students protesting its use, mostly along the lines of inappropriate use of intellectual property (and some about presumed guilt, which I decline to address).
In 2000 I contributed a chapter to a book on U.S. launch vehicle history. Before I sent my final draft to the editors, I ran it through Turnitin. This I did as a form of self-preservation, because given that the vast portion of my chapter was heavily based on other sources, I wanted to be sure that I had done an adequate job of paraphrasing and attribution. (My scan, by the way, came up clean.)
Now, while I have some concern that a copy of my chapter resides on Turnitin’s servers, I am fairly certain that no more than appropriately-sized excerpts will ever be reprinted there, and then only in cases of likely inappropriate borrowing. Moreover, I am happy that it’s there, for this reason: it establishes a record of prior existence. No one subject to a Turnitin review will be able to crib from me without proper attribution.
None of this, of course, has prevented a substantial case of plagiarism of my chapter by a contributor to the Thor rocket page on Wikipedia.