As in taking pix of earth, not just happen to point towards earth during a maneuver or whatever.
Like Gauti's answer, xkcd also links to this question on Hubble's website. Hubble moves too fast for its minimum exposure time to be able to focus on anything on the surface. He goes one step further, linking to this article on Bad Astronomy that points out that Hubble frequently points at the sunlight side of Earth to help calibrate its WFPC2 camera.
Still, you may be surprised to find out that Hubble routinely points at the Earth! It uses the bright, daylit Earth to help calibrate one of the cameras on board. The Wide Field Planetary Camera 2 (WFPC2, pronounced ``whiff pick 2'') is an electronic device which detects light, and is similar in principle to a normal digital camera. However, it is far more sensitive, and astronomers are very particular about how well they understand such devices. They want to make sure that every part of the detector is very well calibrated.
One way to calibrate the camera is to look at some bright, evenly lit object. If one part of the detector is more sensitive than another, for example, then part of that object will look brighter, even if in reality it isn't. This can be used to correct any later images.
In space there aren't any flat fields you can use, but we do have one right here: the Earth. To calibrate WFPC2, sometimes Hubble is pointed straight down at the Earth. As the terrain (or water or whatever) streaks across the field of view it forms a very crude flat field image. The image is called a ``streak flat'', and looks really weird. Trees, houses, all sorts of objects blur across the image. It takes a lot of sophisticated computer processing to turn this into a real flat field for Hubble to use, but luckily there are a lot of smart people working on the problem.
xkcd also shows an approximation of what it would look like if you tried to take a picture of an average desk from orbit. It is just a smear of colors, devoid of any discernible detail. I tried attaching the pictures here, but SE doesn't like the png files for some reason and I don't have time to do the file conversion. Just check out the first link to see what it looks like, plus a more detailed explanation of why.
The surface of the Earth is whizzing by as Hubble orbits, and the pointing system, designed to track the distant stars, cannot track an object on the Earth. The shortest exposure time on any of the Hubble instruments is 0.1 seconds, and in this time Hubble moves about 700 meters, or almost half a mile. So a picture Hubble took of Earth would be all streaks.