Serial # in the firmware. It would be costly to have them printed on the board but it may make sense to add a silk screen label spot for hand writing the number when flashing the PIC. Don't know if that's something vendors would want to do or not. The serial# in firmware is easily viewed on screen. I should also add it in the API stats so it can be used in automated ways.
+1 for serial number in stats. Im thinking of making a different tool to suck stats from cgminer and maintain history per device, and help identify the exact location of a particular device (which may be having errors) by comparing serial number with usb bus/port ids.