since iOS4 IPv6 is enabled by default in iPhones
sadly there is no way to enable privacy extensions for IPv6 nor a way to disable IPv6
with iPhone settings...
so if you want to, you will have to jailbreak your phone just to do that.
so look for a jailbreak for your phone and do it - its really easy...
after jailreak set new passwords for user: mobile and root
(you really do not want a root-account with password alpine)
- install a terminal
- become root
- do the following:
sysctl -w net.inet6.ip6.use_tempaddr=1
- have fun with IPv6