The big question (for me anyway) is whether to implement this as some kind of actual hierarchical storage structure, or just via metadata.
I think that should be up to the repository; the only thing that really matters is the reference scheme (and a perceived hierarchy is as good as any, i.e. /a/b/c).
Problem with collections is that the become circular, which again cause some... issues.
You can imagine a hierarchy as an implicit collection, where the path of the page determines implicitly the main collection they belong to. This is good because it means that you don't explicitly have to tie a page to a particular collection, and does not preclude the idea that pages could then belong to multiple collections.
Hm. Something in here sounds interesting... /Janne
