About an hour ago, I checked in an extension which provides a fairly
simple, but customisable, email notification for new user accounts.
It's designed to allow a number of recipients, and the text of the
email can be customised as one would expect.
See
http://cvs.sourceforge.net/viewcvs.py/wikipedia/extensions/NewUserNotif
to download it; the readme file was checked in about 2 minutes ago,
and isn't in anon. CVS yet, but it is available from
http://meta.wikimedia.org/wiki/User:Robchurch/New_User_Email_Notification
as are those for all extensions I write.
Rob Church