At this time there is a widget to select a tag in a category. I mean, it is possible to define a field that is based on a tag category, the contributor can select a tag in this category.
But the tag is not applyed as a tag on the content. It is stored as a value of the field.
So it is possible to use it in a filter in the skin but not in a content digest with tags, for example.
this widget was not done for this but it could be nice to have this kind of widget.