An external diode to clamp the input from going above the AVR VCC voltage might be a good idea - I wouldn't want to put that kind of stress on the internal clamping diodes (if there are any).
Sure a one amp diode and a series say 100 ohm resistor between the analog pin and the junction of the voltage divider would make it pretty bullet proof.
Yes, an additional resistor in series with the input is a great idea. Even if the external clamp forward drop is a bit higher than the input pin clamp, the resistor will limit the current and prevent a latchup.
The reason I mentioned an external diode is that some microcontrollers have actual clamp diodes on the inputs while others have "pseudo" diodes formed by the junctions of the input FET's. If THOSE are driven above VCC or below ground, the input stage can latch up like an SCR and then the holy smoke
is released from the chip and it doesn't work anymore!
I don't know if the AVR micros have actual clamp diodes on the inputs or not. Better safe than sorry.