See UseMod:SubPage for more information.
See also LinkingBetweenNamespaces.
One issue is dealing with RecentChanges. Probably a change to a sub-page should be folded into changes to the root page, at least for people who don't care. Perhaps RecentChanges needs to be a dynamically expandable hierarchical view, for people who do care, or perhaps each page should somehow act as a local RecentChanges for its subpages. -- DaveHarris