Each card must have a unique identifier, so i thought each card has a S/N (serial number), but is it true?
Even if your arduino has an FTDI, there is no way for the arduino to query itself.
you think the ATMega48/88168/328 have some S/N built into the hardware that can be read somehow?
mega8u (used on Uno) has a serial number, at least by the time it has become a LUFA-based USB/Serial converter.
Pretty much any other usb device would have a serial number.
The new bootloader does not support programming EEPROM from the IDE...