So why is it we think we can do better in our law firm knowledge management programs? The reality is that people often define the "proper place" for content differently. You only have to look at the variations in social bookmarking to see this. So, for example, instead of creating a rigid top-down taxonomy that imposes a regime of a single place for each thing (and then devoting the necessary resources on enforcement), why not spend your energy creating systems that allow users to organize the content as they see fit? After all, the point is to enable their easy use of the content -- it really isn't about ensuring that they find and use that content only in particular places.
At the end of the day, the purpose of the adage of one thing/one place is to eliminate options so that that you always know where to find your keys, your wallet, your cellphone, etc. With the advanced search tools available today, we don't need to worry about this quite the same way when it comes to electronic content. So instead of enforcing a single way of doing things, meet your users where they are. I guarantee they'll be happier -- and then so will you.