I think you're confusing jailbroken with unlocked. You need an unlocked phone to use another carrier's sim and only the first gen iPhone has been unlocked. The 3G has only been jailbroken.
No confusion, the first iphones when jailbroken would allow them to work with any o2 sim card, didn't have to be the iphone contract one that came with it.
I know what unlocking is and that it's not out yet for 3G.