Ontological Hell
Marc is talking about ways to organize ?tags?. Organizing tags in a hierarchy of namespaces (or filtering with manually managed social networks) is just another way of proposing ontological hell. A middle ground between wide open tagging and namespaces would be for tags with ambiguous definition to cite the particular definition in the dictionary to which they refer (?jump; def 2?). WordNet makes this easy; you can reference a common English word generally, or point to a specific ?sense? of that word. But even this level of precision is too cumbersome to see widespread use.
I think the real solution to the grouping/hierarchy problems is to have the computer do the clustering and mining automatically. Since Aristotle, every human attempt at creating hierarchies of terms has been nothing more than an arbitrary clustering exercise anyway (things with fur are different than things with scales, etc.) Computers do this much better than people; especially for data which is already native to the computer. Tags should be able to be clustered based on a combination of person using the tag, content being accessed, other uses of that tag, and so on.