Atmega Spec says IO should not be higher than Vcc + 0.5V.
Diodes are only good for 1mA as well.
External signals should not be applied to IO pins when power is not on.
If it's going to be a regular occurrence:
Signals that are applied to input pins can have series resistors added to limit the current the diodes. Zener diodes can be added to limit the voltage.
A 5V '328P only needs 1uA and 3V for a valid high, so you can protect the pin externally quite a bit with a couple of components.
See the Ruddeduino at www.RuggedCircuits.com to see this taken out to every pin and then some.
http://www.ruggedcircuits.com/ruggeduino