Arduino Mega2560 Hardware ID
VIP/PID is realized by Atmega16u2 driver which implements USB device driver. Probably all revisions of *Arduino Atmega2560*s has implicitely the same VID/PID. Each piece has unique serial number. But you can upload own firmware into 16u2 to modify USB as you wish (ideally using LUFA library). It's clean solution for production hardware because you cannot use Arduino VID because of USB rules.