Thanks, andrea & lunabyte-
Regarding something that may have changed on the server, I would have no idea where to begin looking at that. I looked in varioius logs available to me thru cpanel, but nothing jumped out at me.
And I thought about the notification being rejected on the receiving end, but I've tried using many different e-mail addresses & domains (yahoo.com, gmail.com, berkeley.edu), and none of them are receiving the welcome notification.
My last thought was to modify the content of the username/password e-mail, since that is getting sent & received successfully.
Might anyone know what .php file that e-mail is sent from? It doesn't appear to be wpmu-functions.php, since I can't find the text (e.g., "Your username and password") that are included in that e-mail.
Thanks again for your help.