Angostura, I'm not on the OmniGraffle team, but if nothing else, it should be possible to export from OmniGraffle to OmniOutliner's file format. OO can then export to a tab-delimited text file. (OO can export is in trial mode, if that helps.)
If you email the
support ninjas, there may be an easier way which I'm not thinking of. Hope this helps!