"Tag Bundles" in del.icio.us are quite possibly the Right Way to do hierarchy for tags:
This feature allows you to combine several related tags into a logical grouping. So, for instance, you might combine the tags “hitter”, “pitcher”, and “fielder” into a bundle and call it “Baseball”. It doesn’t change anything about the existing tags, but does allow you to create another level of heirarchy. When looking at your bookmarks, del.icio.us will show this bundle and all the tags grouped under it as a separate section.This is really meta-tagging but explained in plain English with a perfectly sensible immediate end user benefit. Plus bigger benefits down the line if these bundles are shared.