MrMark:
It is good practice to use a series resistor (about 1000 Ohms) between Arudino and RPi GPIO pins to limit current. This protects the RPi in case the Arduino pin is accidentally configured as an output.
Or when the Arduino is powered off.
3v3 as an input is fine 
Using an interrupt is useless unless the code can actually break on different places and the start and stop signals are very fast. Other, just poll using digitalRead().