What if the contents of that DB get pre-rendered to disk or memory for caching (eg: prerendering a bunch of HTML)? Do you blow those away? Which ones? What if it turns out to be a substantial number of cache records you need to blow away? Whats gonna be the performance impact of that?

I agree with the OP. People who assume this shit is easy haven't really thought about the problem much at all. There is a lot of data stored out there in ways that wasn't really designed to be mutable.