I'm a big fan of tagging (folksonomy, as the kids call it), and I've written previously about the need to this be adopted to file systems as quickly as possible. One problem with tagging that comes up is when the existence of one tag implies that others should be applies. For example, "Mac" implies "Apple". This is a formal hierarchy, of course, and that effectively means putting tags in directories: exactly the scenario we want to avoid!
Luckily, the way out of this is simple. Tagging of tags! This follows from our first principle: folders are so 1990's and hierarchies are unnecessarily restrictive. Of course, we wouldn't make a distinction between types of tags; a tag could be applied to another tag or another "entity". So for a post about "Melbourne Business School", I usually use the "melbourne business school" tag. The "melbourne business school" tag should itself be tagged with "university" and "business" and perhaps "study" or "work". Similarly, sometime I write about ColdFusion. ColdFusion should be tagged with "technology", "Adobe" and "software". Potentially, "software" could likewise be tagged with "technology" but that's just fine.
So far, I have only seen this concept applied in one place: Taglocity, an Outlook plug-in, although it is framed slightly differently. I'd like to see more it, especially for blogs.