I found this fascinating quote today. Regarding TurnitIn:
Recycling and repurposing of text online is becoming so ubquituous that the noise is casuing a problem in the interpretation of originality reports. They used to save us time in investigating cases of plagiarism, now I’m not so sure.Nick Sharratt, DrBadgr, Nov 2009
As we are about to use TurnitIn for dissertation theses, this arrives as a timely warning!