Semantic Web Outliner : blog, sourceforge (can't see a release yet). Not exactly an outliner, that's only one of the views, there's a node & arc graph there too. From Martin Dvorak in Prague (I think).
This is very very similar to what I originally had in mind for IdeaGraph, i.e. mindmaps done SemWebbically. That was before I got carried away... (I do intend to return to IdeaGraph in the next two or three months, but simplify and make it more project/goal-oriented).