Reading the thread to this point - there seems to be some confusion. Other company SIM card cannot be used in the iPhone. The iPhone SIM card can be used in an unlocked phone from any company.
The reason other company's SIM cards cannot be used in the iPhone is (most likely) because the iPhone is locked to ATT's network. If the iPhone can be unlocked at some point, then other SIM cards should be able to work in it. The key is that it's not something unique about the SIM card, it's the lock on the phone that is the issue.