As someone else mentioned in the thread, make the email address optional, with the understanding that without it they can't recover lost passwords. This is pro

As someone else mentioned in the thread, make the email address optional, with the understanding that without it they can't recover lost passwords. This is probably an unacceptable solution for some sites, but I can definitely think of cases where the only reason you need email is for password recovery, and account loss isn't the end of the world.