Originally Posted by
Playboy
First, a prepaid sim will only work if you've unlocked your Iphone 4. That goes for any GSM phone. And the only way to unlock it without getting the unlock code from AT&T, which they likely will not give you, is to jail brake your Iphone and then unlock it.
In France, anyone can get any phone legally unlocked after 6 months of service with a wireless company. I assumed the OP, coming from the UK, could have access to similar regulations.