The raspistill command offers at least 30 command line options but no option to capture a grayscale image. There are considerable advantages to that mode for using a Pi as a security camera.
1. All of the BW info is contained in each single 8-bit pixel (looking at the Green value only gives you 70%).
2. The BW image takes up 1/3rd as much RAM and when you save an image to the filesystem the same JPEG quality is 1/10th the size of the RGB image.
Here's my little bw.py program (which assumes you are passing it an RGB file name):
from PIL import Image
a = len(sys.argv)
if a != 2:
print "Usage: bw name.jpg\n"
im = Image.open(sys.argv).convert("L")
file, ext = sys.argv.split('.')
im.save(file + 'BW.' + ext, "JPEG")