Tom, mixed case usernames are allowed, but we treat them case-insensitive, meaning you cannot have two users with the same username except for case, and we don't care in which case you enter it. However, we do store it with the case with which you entered it.
Hm. Just now I'm realizing that when passing the username on to the authentication driver, we use the case the user entered when logging in, not when registering, which may be a problem with authentication services that happen to be case-sensitive.
/Lars
