OK I know some people still are confused on this so I am doing this for the benifit for everyone. There is no speculation here. This is all based on official Apple and AT&T info.
This is the SIM Tray...
As you can see from my hard labor in MS Paint the tray is located between the headphone jack and the standby button. There is a small hole that you insert a paperclip or similar object in to release the tray.
Now...some people seem to be thrown off on the whole "preinstalled" text. All this means is that the card was put into the tray when the iPhone was made. It is a normal removable SIM that you can use in any other GSM phone. The following was taken directly from the AT&T Training Guide posted on Boy Genius today...
Hopefully this clears everything up once and for all. If not then Friday will tell you the same thing I just did.
This is the SIM Tray...

As you can see from my hard labor in MS Paint the tray is located between the headphone jack and the standby button. There is a small hole that you insert a paperclip or similar object in to release the tray.
Now...some people seem to be thrown off on the whole "preinstalled" text. All this means is that the card was put into the tray when the iPhone was made. It is a normal removable SIM that you can use in any other GSM phone. The following was taken directly from the AT&T Training Guide posted on Boy Genius today...
- SIM cards are not required for repairs, and should not be returned with the device. Advise the customer to remove the SIM card, prior to sending it to Apple, and store it in a safe location.
- If a customer decides to insert their iPhone SIM card into another AT&T device they will have the following experience:
Visual Voicemail will not be available, however regular voicemail will be accessible by pressing “1” to hear messages.
MEdiaNet will be used to access the Internet. MMS and IM messages are included in the customer’s iPhone Data Plan.
Contacts will not be accessible via the SIM.
- Contacts added to the SIM while in another device will not be synced back to iPhone.
Hopefully this clears everything up once and for all. If not then Friday will tell you the same thing I just did.