Arduino schemaic

Ah, the reset pin doesn't have an internal protection diode to limit the input to +5V because +12V is required on the reset pin to enter the parallel programming mode of the ATmega168...

See pg 4 here:

They don't explain this in the data sheet for the Atmega168, they just make a sweeping statement that "All I/O pins have protection diodes to both V and Ground as indicated in Figure 13-1"