Are usernames with an "@" in it not supported?
@ is not allowed in usernames (unfortunately).
This is because @ is used as a separator between username and database name when dealing with multi-wiki installations... In theory you could hack it out and hope nothing goes wrong if you're not doing cross-wiki user permission settings. :)
It would be nice to use this character for domain based users, but I'm not worried enough about it to hack it out. It's mostly other people who request this; I guess they can send in patches if they want it enough ;).
V/r,
Ryan Lane