French jurist said:Yes but you need to jailbreak and unlock it.
If you have an iphone 3GS bought very recently, you cannot jailbreak it for now (Apple implemented a new bootrom aimed at stopping jailbreaking, only the first 3GS that were sold can be jailbroken at this time. Previous models can be jailbroken).
Depends on where you come from, in Belgium and Italy the iphones are not locked...