Maybe this is obsolete because of UTF-8 em dashes?  Originally by KragenJavierSitaker.

    (defun insert-em-dash ()
      "Insert a TeX-style em-dash '---' with appropriate spaces around it."
      (interactive)
      ;; ensure we have a space before the em dash
      (if (not (looking-back " "))
          (if (looking-at " ") (forward-char 1) (insert " ")))
      (insert "---")
      ;; ensure we have a space after the em dash
      (if (looking-at " ") (forward-char 1) (insert " ")))
    (global-set-key [(meta _)] 'insert-em-dash)