I use motion on pi, if I want to run raspistill, I have to stop motion(which also using the camera to detect movement).
pi@raspberrypi ~ $ sudo /etc/init.d/motion stop
[ ok ] Stopping motion detection daemon: motion.
pi@raspberrypi ~ $ /usr/bin/raspistill -o cam2.jpg
pi@raspberrypi ~ $ sudo /etc/init.d/motion start
[ ok ] Starting motion detection daemon: motion.