From The Apple Wiki

This is the hardware chip ID of the baseband NOR memory which contains both the Baseband Bootloader and Firmware. Its size is 64 bit for the iPhone 3G baseband and 128 bit for the iPhone 3GS baseband. It's supposedly read-only data that has been burned in during production, e.g. it cannot be changed. The term NOR comes from "Not OR" gates, which are used to create the flash memory.