Adaptation of code from (http://thorbek.net/online/2014/05/27/a-simple-motion-detection-program-for-raspberry-pi/). I have changed the code to get the motion detection frames from the video port at a lower resolution, and to snap a high resolution shot when motion is detected.
This allows me to capture most movement in front of the camera at a high resolution (bearing in mind that it can take 750 ms to get a good quality still capture), but with relatively little cpu load (the pi load average is around 0.40)