Are you asking about the concept or the labeling?
For the concept, there are valuable "identity" consideratinos from
social design patterns at
http://habitatchronicles.com/2008/10/the-tripartite-identity-pattern/.
For the labeling, I understand "username" as authentication, and
"screenname" as display, which despite bound to the same person, must
be kept separate for
many reasons you can read on that link above.
Good luck!
Gino