Daniel Garcia from Protostack, the guy who sent us an ATmega8 dev board a while ago for review, wrote a tutorial on how to use the atmega168 external interrupts. The tutorial is quite nice and you should know the way around external interrupts at the end of it. The general principles apply to other AVR microcontrollers, but the specific vary greatly. Remember that we also have a tutorial about I/O handling on their dev board.
ATmega168 external interrupts: [Link]