I have posted some thoughts on using DITA for narrative documents on the Really Strategies blog: http://blog.reallysi.com/2008/01/dita-applicatio.html

The executive summary is:

By ignoring the existence of the <section> element you quickly realize that nested topics can be used just fine to represent narrative documents with arbitrarily deep nesting of divisions. This potentially opens up a whole class of documents of particular interest to Publishers that might otherwise not seem to be good candidates for ditification.