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ECID
The Exclusive Chip ID Tag or ECID, is a 3GS device specific numeric identifier which is sent to Apple during a restore. The ECID is included in a new, device specific, signed img3 file and sent back to iTunes for the restore process to proceed.
The implication of this is Apple could dissallow downgrades (even via DFU) on newer devices and even on older devices once new firmware is released.
