Hey Great project! I have a write up about a similar project posted on my blog that friend just finished. He had the same problems but ended up tackling them in different ways. You might want to take a look since the project is so similar.
http://n0m1.com/2012/05/13/mday-low-power-motion-sensor-arduino-night-light/
We wrote an arduino sleep library that was used in the project. It has both wake from interrupt as well as watchdog sleep. Its here:
I hope this helps someone out!