Arduino Atmega8 vs Atmega168 pin mapping

Does anyone know to what extent the Atmega168 and Atmega8 have different PDIP pinouts? I'm noticing a strange behavior with my Arduino board when I use Atmega168 vs Atmega8. When I set Arduino's Digital Pin 13 to input, I can use it that way successfully if the processor is an Atmega8. But if it's an Atmega168 it always reads as 0. Pin 12 works fine with both processors. Now it's possible I have a defective Atmega168 - but I've seen this behavior with two Atmega168s.

I just looked at the Arduino USB schematic http://www.potemkin.org/uploads/Pid/arduino_usb.jpg and see that there is a 1K resistor only on Digital Pin 13 - perhaps the Atmega168 is more sensitive to its effects than the Atmega8.