Hi Bruno,
With the Iphones or other smart phones, it would be advisable to completely shut the phone down and reboot. This should cause the phone to flush it's local cache of DNS etc... Are you having the issues over 3G connection or through Wifi and in which case which operator / ISP. Just last week, O2 implemented a content filter in the UK, which started blocking adult content. It requires the customers to phone them and have it removed.
Hope this info helps.
EN