Does nobody read anything before they post anymore!!
These questions are being asked over and over again all over the forum and many of them are already answered at apple.com!
There IS a SIM card slot/tray, but no other SIM will work other than that whish is registered with that handset, even another valid AT&T SIM from another iPhone will not work in another iPhone.
Actually, I have read that another ATT SIM card would work, but Apple prefers that you use the SIM card that came with the iPhone. The key here is the locking of the iPhone -- that is what will usually prevent another SIM card from working in any GSM phone. If the iPhone can be unlocked (and I strongly suspect that it can be, as there are news reports saying folks are already doing this in India), then any SIM card should work in the iPhone. But I would strongly recommend to anyone that they wait and see how the unlocking process works first before buying an iPhone with the hope of using it on a non-ATT network.
I fully expect that within a few months I will be able to purchase one and use it on T-Mobile in the US and Orange in the UK, but I also want to wait and see if anyone develops modem scripts for the iPhone so that it can be used to transmit data to/from my MacBook Pro.