From Quick Guide: Cell Guide
Summary: To find the IMEI number on a mobile phone, turn the phone off, remove the battery cover, remove the battery, and find a long string of numbers above a bar code. Locate the IMEI number, which appears on TMobile and AT&T Cingular phones, with information from a wireless specialist in this free video on cell phones.
"I'm Ben from the Cellphone Broker in San Jose California and today I'm going to show you how to find the IMEI number, I M E I number on your phone. The IMEI is on one particular type of phone and that would be GSM phones which in the United States is TMobile, or AT&T Cingular. The IMEI number is the serial number for that device and the carrier, AT&T or TMobile, uses to track it. So what you're going to do is you're going to make sure your phone is turned off. You're going to turn it around wherever the battery is. You're going to remove the battery cover. Sometimes, there's a button. Sometimes you can just slide it off. You're going to remove the battery, very carefully and your IMEI number is going to be right there on the top, that top bar code right there. And it's got a pretty long string of numbers there. Just make sure it's not a combination of numbers and letters. It's only numbers. So there it is. I'm Ben from the Cellphone Broker in San Jose, California. You can check us out online at myspace.com/cellphonebroker."
eHow Article: How to Find the IMEI Number on a Mobile Phone