Sorry, I misunderstood where Brian was probably measuring. I assume he has checked the operation separately. It is almost certainly the series resistor is too high, as others have said. If the sensor unit pulls the 470ohm load to ground if used in a standalone circuit (disconnect from P3.5 pin, and operate slowly) then with a 100ohm series resistance for protection (u do use protection, right :-) ? ) then it should also haul the pin node to a logic low too.