One way could be using two standard 5V regulator circuits (one circuit for one light barrier) based on a 78m05 and three capacitors, which should regulate the 12V (object detected) down to 5V.
No pull-down resistor required, the voltage divider is also acting as a pull-down as the lower resistor is wired to ground and is why the arduino ground must wire to the divider and signal source's ground. As far as what a legal HIGH voltage is the AVR data sheet says minimum voltage for a HIGH is .7 X Vcc, so with an arduino 5volt board the value is 3.5vdc to 5.5vdc.Lefty
Good to know! Can this be found on the Arduino playground? If not, it should be pinned here or elsewhere.