OHO! I bludgeoned it into submission - and yes, my solution sucks, I'm sure.
jQuery("#user_webmail").blur( function(){ var un = jQuery("#user_webmail").val().split("@"); jQuery("#user_webmail,#user_pay,#user_home").val(un[0]); } ); Any cleaner solutions are highly appreciated. Now, I just need to read up on how to keep my image submits inactive until both a username and password are filled out.