Apple iPhone OS 1.0 through 2.2.1 and iPhone OS for iPod touch 1.1 through 2.2.1 stores an exception for a hostname when the user accepts an untrusted Exchange server certificate, which causes it to be accepted without prompting in future usage and allows remote Exchange servers to obtain sensitive information such as credentials.
https://exchange.xforce.ibmcloud.com/vulnerabilities/51208
http://www.vupen.com/english/advisories/2009/1621
http://www.securityfocus.com/bid/35447
http://www.securityfocus.com/bid/35414
http://support.apple.com/kb/HT3639
http://lists.apple.com/archives/security-announce/2009/Jun/msg00005.html