Rowan Collins:
One thing that I was pondering when you posted about this before is whether the "special behaviour" of namespaces like "Image:", "Category:", etc could be built into some kind of hooks mechanism.
Yes. These kinds of hooks are essential to Wikidata and Ultimate Wiktionary (especially the latter). It will be possible to attach different application classes to different namespaces. Wikidata will be one such application, providing versioning, search and user interfaces for arbitrary structured data. The methods in these classes will themselves contain hooks for extensions and customizations.
Besides reducing some of the "magic" of namespaces like NS_IMAGE, and the ability to add arbitrary structured content, Wikidata will have uses even within the default namespaces. NS_IMAGE is an excellent example. It is currently a table containing versioned structured content (the image metadata) that is, however, completely separate from the image description page (and still split in a CUR/OLD way). With Wikidata, this would just be a customized wiki page with some added fields and special hooks,
The best part is that the Wikidata design allows for translations, so you could add something like a default caption to the image page as a separate field, and translate it into all languages. When you type [[Image:Foo.jpg|thumb]] anywhere, it would then use the default caption in whatever language is appropriate. This could also be used on category galleries. And I've posted here before on the uses of Ultimate Wiktionary to translate category names. Commons will therefore be one of the major beneficiaries of Wikidata, with the goal being a move towards powerful, automatically localized tagging as the main method of organizing content.
But even this just scratching the surface. Wikidata and UW are hugely complex projects with enormous potential, if done right. Because they are so complex, we are trying to implement milestones which are independently useful. The namespace manager is one example, more will follow. I'll post further details soon.
One note on namespaces: The namespace manager is _not_ intended to add hundreds of namespaces to a wiki. Namespaces are for very large sets of pages (or special functionality). The Cookbook: namespace on Wikibooks is one example, the abandoned Jokebook: was another - but most books on Wikibooks are probably too small to deserve their own namespace.
Some people will doubtlessly create wikis with hundreds or thousands of namespaces, and run into some limitations. These may be addressed, but in the Wikidata context, we're typically dealing with few namespaces. For example, Ultimate Wiktionary, in spite of having a huge set of tables, will probably only have one or two namespaces. There are additional ways to restrict a set of content, and one idea I'm playing with are link parameters. To get just the "animal" definitions of the word dog in Ultimate Wiktionary, for example, you could use something like [[en:Dog|related to=Animal]].
Erik