You don't have to tell AT&T the IMEI of the iPhone 3G. It will self-report everytime you connect to the AT&T cell network using your SIM. Just put the SIM into the iPhone and wait about 15 minutes, then log in to OLAM and you will see the iPhone 3G listed as your device on the line. Yes, you can manually change it in OLAM but that just customizes the help and feature support, it does nothing to change the auto report feature everytime you connect to the cell network.
Dave