ImageGrab from PIL would have been ideal. I'm looking for similar functionality, specifically the ability to define the screenshot's bounding box. I've been looking for a library to do so on Mac OS X but haven't had any luck. I also wasn't able to find any sample code to do it (maybe pyobjc?).
While not exactly what you want, in a pinch you might just use:
os.system("screencapture screen.png")
Then open that image with the Image module. I'm sure a better solution exists though.