With the UNO, the Arduino team used a reprogrammable ATmega8/16u USB chip using standard CDC USB-to-serial interface, which all OSes support natively. Unfortunately, on Mac OS X, the OS’s CDC driver creates a serial port for the UNO based on USB port location, not Arduino serial number.

Thus where you plug in the UNO into determines its serial port name. For instance, on my Mac, plugging in an UNO with serial number “64936333936351408161″ gives a serial port with the name of either “/dev/tty.usbmodemfd131” or “/dev/tty.usbmodemfa141” depending on which USB jack is used. Even if you plug in a different UNO in the same jack, you’ll get the same serial port name. This is useful for some applications, but not if you need to know exactly which UNO you’re dealing with.

On the larger series 6 and 7 USB AVRs (AT90USBxxx6 and AT90USBxxx7) there is a unique serial number embedded within the signature row, for just this purpose. Until now, there hasn’t been a good way to extract this serial and use it in the USB descriptors. However, with the new release, on compatible AVRs, it is simply a matter of setting the serial number descriptor index to the new magic USE_INTERNAL_SERIAL< value. That is:

USB_Descriptor_Device_t PROGMEM DeviceDescriptor =
{
        //...
	.SerialNumStrIndex      = USE_INTERNAL_SERIAL,
        //...
};