I am wondering if a taxonomy structure exists for the web users in DNN.  For instance, if there are some additional attributes (apart from regular user profile) that need to be tracked for a user, such their topics of interests (what they like - sports activities, business, science journals, etc), or other simple attributes such as whether they would like to receive notifications, and if so, by email or regular mail, and if email, HTML format or plain text ... things like that.

Our own system's data structure has this type of taxonomy liked with users, and we would like to integrate our structure with something similar in DNN, so that when these attributes are updated on the users profile in our system, they also get updated on DNN side automatically.  If DNN has nothing like this, is there some application like this that someone has built and would like to share?

Thanks! 

It is coming and fairly soon.  There's not much yet on Taxonomy aside from:
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=11564&PROJID=2
and (briefly)
www.dotnetnuke.com/Community/Blogs/tabid/825/EntryId/2459/Community-2010-A-Building-Wave.aspx
but I suspect once they get it working they will then try to document it.