That was actually being done on a site where the entire signup process was customized, so I don’t really have a code snippet or anything that I can share I’m afraid. If I was doing it now, I’d attempt to hide the username field, and then when the sign up form is submitted, generate a username based on a hash of their email address. I think you could inject some JS code on the ‘signup_extra_fields’ action to hide the username field, then before/during the processing of the form, you’d have to overwrite $_REQUEST[‘user_name’] and maybe $_POST[‘user_name’] as well with your generated username. HTH.