Support for

I added support for hierarchical taxonomies so that the resulting single select would show optgroups for new sub categories. See the Diff file for the changes. Enhancements could be made such as adding an options page with an option to make sub categories selectable instead of using optgroups. This works for what I needed it for so I won’t be adding this enhancement but if more want it I’m happy to help. Cheers http://wordpress.org/plugins/single-value-taxonomy-ui/