I was happy to see that Technorati now automatically maps from the category system in many common blog systems to tags:
“If your blog software supports categories and RSS/Atom (like Movable Type, WordPress, TypePad, Blogware, Radio), just use the included category system and make sure you’re publishing RSS/Atom and we’ll automatically include your posts! Your categories will be read as tags.”
One might like to have tags that differ from the blog category system (e.g., finer grained), but I think this might give me 80% of what I want with virtually no effort. For example, here’s what Technorati has under the SemanticWeb tag. We’ll have to re-think our categories, add some new ones, and get in the habit of categorizing a posting more carefully.
Blog category systems, at least the one that comes with WordPress, are hierarchical. If the AI category has a subcategory MachineLearning, then all postings tagged with MachineLearning will show up when you look at the AI postings. So, we may have to do this manually in order to have that posting show up on Technorati under both tags, AI and MachineLearning.
Another problem is that the names in a category system are often chosen to make sense in the context of the hierarchy, differentiating the category from its parent, siblings and children. In a flat and more open tagging environment, there is no contextual information, so that the names have to carry all the weight.