Use `starttls-available-p' to see whether starttls.el can be used.

lisp/ChangeLog

+2011-08-02  Lars Magne Ingebrigtsen  <larsi@gnus.org>
+
+	* net/network-stream.el (network-stream-open-starttls): Use
+	`starttls-available-p' to see whether starttls.el can be used.
+
 2011-08-01  Martin Rudalics  <rudalics@gmx.at>
 
 	* window.el (display-buffer-in-window): Don't set dedicated status

lisp/net/network-stream.el

@@ -230,7 +230,7 @@ functionality.
 	       (or builtin-starttls
 		   (and (or require-tls
 			    (plist-get parameters :use-starttls-if-possible))
-			(executable-find "gnutls-cli")))
+			(starttls-available-p)))
 	       (not (eq (plist-get parameters :type) 'plain)))
       ;; If using external STARTTLS, drop this connection and start
       ;; anew with `starttls-open-stream'.