;; Courtesy of offby1
(defun my-rename-file (bufname newname)
  "Rename the file visited by buffer BUFNAME to NEWNAME.  BUFNAME
defaults to the current buffer.
This is a bit like \\[dired-do-rename], except that you don't have to
run \\[dired] to get at it!"
  (interactive "bBuffer: \nFNew name: ")
  (let ((oldname (buffer-file-name)))
    (or oldname (error "%s isn't visiting a file" bufname))

    (if (file-directory-p newname)
        (setq newname (concat newname (file-name-nondirectory oldname))))

    (rename-file oldname newname)

    ;; TODO -- ensure that, if set-visited-file-name fails, the file
    ;; rename gets undone.
    (with-current-buffer bufname
      (set-visited-file-name newname nil t))
    (message "Renamed %s to %s" bufname newname)))