From page 304 or section 28.1 of the ATmega168 datasheet, the ATmega is rated to it's supply voltage +0.5V on any pin WRT ground.
Stresses beyond those listed under “Absolute
Maximum Ratings” may cause permanent dam-
age to the device. This is a stress rating only and
functional operation of the device at these or
other conditions beyond those indicated in the
operational sections of this specification is not
implied. Exposure to absolute maximum rating
conditions for extended periods may affect
You can ofcourse use a simple voltage divider to divide the 5v signal into something in the 3.3 volt range. Obviously the tolerance of the resistors will impact the accuracy of the ADC.