The Omni Group has released version 3.2 of OmniGraffle and OmniGraffle Pro, Mac OS X based drawing and diagramming tools.
OmniGraffle is my tool of choice for drawing UML diagrams on the rare occasions when I need to do that.
Version 3.2 adds support for importing OmniOutliner 3 files.
OmniGraffle 3.2 is $69.95. The pro version is $119.90. Mac OS X 10.2
or
later is required.