Who is responsible for detecting plagiarism in a PhD thesis? Background
I am asking this question in the context of the long running plagiarism scandal in Germany.The most prominent victim of the scandal of plagiarized doctoral dissertations by politicians was former defense minister Guttenberg. His doctoral title was withdrawn, and he resigned from his position. He was followed by several other politicians, and now even the German Education and Research Minister Annette Schavan is under suspicion of plagiarism.

The interesting thing is that for all these people, the plagiarism was detected only due to plagiarism detection wikis like VroniPlag and GuttenPlag, where ordinary people compared these doctoral dissertations with other published work. The only role of universities has been to respond to allegations that are especially well documented and have caused a reasonable amount of public uproar.

Don't the Universities themselves bear any responsibility for letting this plagiarism go undetected? In fact Guttenberg's dissertation was awarded a summa cum laude. It appears as if their is no mechanism whatsoever (at least in German universities) to detect academic fraud.

Is there any mechanism at all to detect plagiarism or fraudulent research in PhD dissertations (doctoral theses)?

I understand that there are probably country based differences and my experience is primarily with the German system. Still it would be good to know the seriousness with which academic fraud is taken in different countries. This appears especially pertinent to the maintenance of the credibility of academia in general and doctoral degrees in particular, and yet there seem to be no checks whatsoever!
The problem is that the PhD system is designed for people who intend to become researchers.  For these cases, plagiarism is not at all a common problem.  You are expected to published your research, and you will not have a successful career unless it is widely read and cited.  That gives lots of opportunities to get caught, and the penalties for plagiarism are a huge deterrent.

To the extent you find plagiarism, it's generally people who do not want a research career, but instead view the PhD simply as an obstacle on the road to a teaching (or other) career.  Probably the community should scrutinize these sorts of theses more carefully, but it can be hard to work up the energy to do so when most of them are OK, and when these theses really don't matter much for the research world.

The German politicians are pretty much the worst case scenario.  In the US, the stereotypical case is educational administrators.  Typically, you have a distinguished person who starts to feel the need for a PhD.  Perhaps it's because they associate with academics and feel looked down upon, or perhaps it's because an academic endorsement would make the public value their expertise more.  This student is very smart and accomplished, and nobody suspects them of any dishonesty.  However, they are also very busy, often working on a PhD while pursuing other projects as well, and academic research is not a priority.  At some point, they succumb to pressure and start taking shortcuts.  Probably it starts with small things, but the shortcuts gradually grow larger.  They rationalize that the thesis doesn't really matter anyway, because they have no intention of following an academic career track.  After all, they have the knowledge and experience, and they deserve the PhD title, so what difference does one document make anyway?  Meanwhile, the advisor probably doesn't spend that much time working with the student, and has no reason to suspect anything.  The advisor really ought to be extra careful in cases like this, but that would seem like an insult to the student, so it's easiest just to trust them.

So my take on this is that plagiarism is not as widespread as news stories might suggest.  It's just particularly likely to happen in cases where it would attract media attention.