Wireframes Mag pointed to an XML to OmniGraffle Sitmap Generator created by Jason Kunesh at Fuzzy Math to generate a sitemap from an XML file. The script assumes that your XML file consists of url elements, and that parent child relationships are defined by the paths in the url. For instance, //konigi.com/about-site/unicorns would be a child of //konigi.com/about-site. Once you generate a list of your urls and pass it to the script, OmniGraffle generates the sitemap hierarchy for you.
http://blog.fuzzymath.com/2010/08/omnigraffle-sitemap-generator-from-sitemap-xml/