I have been telling all the journalists that call me for some time now that I don't believe in solving social problems - in this case, plagiarism at university - with software. It gives the illusion of a solution without actually doing anything about solving the basic problem, namely, that many students do not have any idea how to do research or how to write up something.
I found this blast against turnitin, the multi-million-dollar plagiarism detection service that conviniently keeps copies of all of the material offered to it for testing, so that it has more good stuff for its database. Scroll down to the scenarios - these are not just made up, I had a similar story reported to me just the other day.
A student had turned in a pre-version of his paper to his professor, who checked it against turnitit - no problem, seemed original. When the paper was finished, he turned it in to the office. Official policy was to run a check before giving the paper to the professors to read and grade - and guess what, the paper now registered as a "high-probability of plagiarism" - because it was very similar to the first draft stored in the database.
I don't want to repeat the article here - just get over there and read the article yourself, following all the links to convince yourself that this is not just an angry competitor writing.