Apple iPhone OS 1.0 through 2.1 and iPhone OS for iPod touch 1.1 through 2.1 changes the encryption level of PPTP VPN connections to a lower level than was previously used, which makes it easier for remote attackers to obtain sensitive information or hijack a connection by decrypting network traffic.
http://www.vupen.com/english/advisories/2008/3232
http://www.securitytracker.com/id?1021269
http://www.securityfocus.com/bid/32394
http://support.apple.com/kb/HT3318
http://secunia.com/advisories/32756
http://lists.apple.com/archives/security-announce/2008/Nov/msg00002.html