Woah! I'm glad I came across this before we upgraded to the latest release in production. Joe, did you have a particular business reason for disabling this key? The reason I ask is that (as you know) we have folks auto-registering via Active Directory. For one of our sites (so far) the administrator is using that new user notification email as a prompt to go in and assign appropriate security role(s) to the new user. This is a site where we can't auto assign these roles, since the role is based on the user's division within the department.
Any chance you could restore the use of this key, or better yet add new user admin notification as a per-site option in the settings?
Thanks,
Jamie