Mail.app in Mail for Apple Mac OS X 10.3.9 and 10.4.2 includes message contents when using auto-reply rules, which could cause Mail.app to include decrypted message contents for encrypted messages.
http://www.ciac.org/ciac/bulletins/p-312.shtml
http://www.auscert.org.au/5509
http://secunia.com/advisories/16920/
http://lists.apple.com/archives/security-announce/2005/Sep/msg00002.html