As you can see in the official MS powerpoint slides here: http://www.microsoft.com/germany/msdn/launch2008/library.aspx?id=SP_T16_DI_1800 it is possible to program a hierarchical view of sharepoint wiki pages.
i searched the web and i only found blogs full of the known weaknesses of SP wiki like: no comments, no pictures, no mark up language, no full text search etc.
but this is a thing which should be possible somehow, i hope.