Toggle between root and non-root users

toggle-root.el

(defun find-file-as-root ()
  "Find a file as root."
  (interactive)
  (let* ((parsed (when (tramp-tramp-file-p default-directory)
                   (coerce (tramp-dissect-file-name default-directory)
                           'list)))
         (default-directory
           (if parsed
               (apply 'tramp-make-tramp-file-name
                      (append '("sudo" "root") (cddr parsed)))
             (tramp-make-tramp-file-name "sudo" "root" "localhost"
                                         default-directory))))
    (call-interactively 'find-file)))

(defun toggle-alternate-file-as-root (&optional filename)
  "Toggle between the current file as the default user and as root."
  (interactive)
  (let* ((filename (or filename (buffer-file-name)))
         (parsed (when (tramp-tramp-file-p filename)
                   (coerce (tramp-dissect-file-name filename)
                           'list))))
    (unless filename
      (error "No file in this buffer."))

    (find-alternate-file
     (if (equal '("sudo" "root") (butlast parsed 2))
         ;; As non-root
         (if (or
              (string= "localhost" (nth 2 parsed))
              (string= (system-name) (nth 2 parsed)))
             (car (last parsed))
           (apply 'tramp-make-tramp-file-name
                  (append (list tramp-default-method nil) (cddr parsed))))

       ;; As root
       (if parsed
           (apply 'tramp-make-tramp-file-name
                  (append '("sudo" "root") (cddr parsed)))
         (tramp-make-tramp-file-name "sudo" nil nil filename))))))

I created a fork which makes a round-trip to and from root on "/etc/passwd" work, at leas for me on Emacs 24.0.50.1 on Ubuntu 10.10.

Excellent, thank you.