--- email_registration.module	2010-07-17 19:53:03.000000000 +0200
+++ email_registration.module.new	2010-07-17 19:56:55.000000000 +0200
@@ -14,6 +14,11 @@
 function email_registration_user($op, &$edit, &$account, $category = NULL) {
   switch ($op) {
     case 'insert':
+      // Don't create a new username if one is already set.
+      if (!empty($account->name)) {
+        return;
+      }
+      
       // Other modules may implement hook_email_registration_name($edit, $account)
       // to generate a username (return a string to be used as the username, NULL
       // to have email_registration generate it)