I've been waiting for this functionality for a very long time and I truly hope the folks at Omni include this as a feature in an upcoming release. Until then, we can thank the fine people at UNITiD for this AppleScript.
omnigraffle
Creating a Table of Contents in OmniGraffle with AppleScript
Omnigraffle Sitemap Generator from Sitemap.xml
A handy little script that will walk through a site's sitemap.xml and map it out in OmniGraffle.