HOW TO DISABLE A STOLEN IPHONE

To check your Mobile Phone's IMEI (International Mobile Equipment Identity), key in the following digits on your phone: *#06# and the IMEI 15-digit code will appear on the screen or check it in Settings/About/IMEI or it is imprinted on the back of your iPhone. This IMEI number is unique to your iPhone. Write it down and keep it somewhere safe. 

If your iPhone gets stolen, you can phone your service provider and give them this code. They will then be able to block your handset so even if the the thief changes the SIM Card, your iPhone will be totally useless. You probably won't get your iPhone back, but at least you know that whoever stole it can't use/sell it either. If everybody did this, there would be no point in people stealing mobile phones. The above listed information was printed in the Contra Costa Market Place, February 2008, (pg. 50-51). 
I've spoken with AT&T to verify this procedure and they've confirmed that they can disable the iPhone as well as the phone number, until you get a replacement phone