Michael Schmidt asks,
"When having a classroom discussion about relatively open-ended things like ethics, how do you signal to students that you value their contributions, while at the same time move the discussion in a different direction (ethical issues in science) from what they seem to want to discuss (the culpability of the German population under the Third Reich)?"Oooh, a technical pedagogical question.
The underlying concern here, that students will stop participating if they don't feel safe in speaking, if they don't sense that their contributions will be validated as meaningful (even if they aren't) is a real one. Students are under immense pressure in the classroom. Grades are part of it, but only a small part. Those aren't just their classmates sitting around them: they're the people who will decide whether they get into a given fraternity or sorority, they are the people they hope to sleep with, they are the their entire peer group with whom they spend 24 hours a day, not just three or four hours a week and seeming too dumb or too smart is a risk.
The key to overcoming the worry voiced here requires us to enlarge the frame. The response to the off-topic comment will exist within a larger classroom context. Just as your best friend can tell you to STFU and you won't get offended, so too establishing a certain type of environment in the classroom is crucial. I do it two ways. One is with humor. It relaxes people. If you show that you are both deeply passionate about the subject and willing to laugh while discussing it, it sets a certain tone in which you can take things seriously without being completely serious. I also ask for questions, auto mechanics to quantum mechanics and use that time to show that I can and will take any comment or question seriously no matter how stupid it seems on the surface, showing it to have deeper implications.
By establishing this sort of feel to the room, a student will be less likely to internalize pulling the discussion away from their point back to where you wanted it to go. They won't feel slighted or called out for being stupid if they are more relaxed.
But then there's the question of dealing with the off-topic comment at the time. Two techniques that I use. The easy one is the extended dismissal. You want to say that while this is an interesting question, this is not a point we should be talking about right now. Notice that statement has two parts. First validate, second steer. Expand the validation by repeating the insight in new terms (a couple of them technical) and say in a sentence how this is connected with a live or classical and important debate. Then say, "This is actually something we talk about for weeks in philosophy 2XX, which is a great class. But I have something up my sleeve, I want to narrowly focus on this question."
The second technique is my favorite. It's intellectual improv, the Kevin Bacon approach, where you take what seems like a random comment and as seamlessly as possible make it appear to have been germane to the conversation the whole time. To use the example in the question, I would take the comment about the Third Reich and relate it to ethical issues in science by discussing the Nazis attempts to brand the theory of relativity as Jewish science and then we have a case study for the ethics in science point I originally wanted discussed. (Of course, that would be an easier step for me since I am writing a book on the subject and may just have some happy, happy news about it to report in the next couple weeks -- he types crossing his fingers, knocking on wood, and hoping that Gwydion doesn't use this as an excuse to start talking about superstition again...)