CalDAV in Apple iOS before 5 does not validate X.509 certificates for SSL sessions, which allows man-in-the-middle attackers to spoof calendar servers and obtain sensitive information via an arbitrary certificate.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2011-3217
http://support.apple.com/kb/HT4999
http://lists.apple.com/archives/Security-announce/2011//Oct/msg00001.html