Really, usernames only have to be unique within a single thread, don't they?
You could get away with something as simple as incremental ids in that case - user1, user2, etc.
You could get away with something as simple as incremental ids in that case - user1, user2, etc.
Edit: for better readability it could be further mapped into a table of human readable handles, similar to how Google does the "Anonymous Lemur" thing in gDocs.