please forgive the cross posting
I am always getting a lot of plagiarism in my courses, especially those in which I require Asians to write in English. I think that, as Bakhtin pointed out, since we are always using the words of others, the students think it is okay to use the expressions of others. It would be if they cited.
Anyway, it is usually quite easy to check using google. I just put in a short 4 word or so phrase and and after a few tries I find out the source material. But it is a very dull and depressing task, the sort of task that I would like to leave up to a computer.
If I were a programmer I think that it would be fairly easy to write an application like Eve2 (which has issues, so I don't recommend it) in php to check some or even all 4 or 5 word phrases to see if they result in Google hits and then to store those hits (or at least one of them) and highlight those parts of the submitted text that can be found on the web and where.
I would be able to donate towards the creation of this sort of functionality (if it worked with moodle 1.6).
Tim