On October 3rd, 2007, they turned it around to look at Earth and got the following image (courtesy of NASA/JPL/and the University of Arizona...click to embiggen).
Since Earth is closer to the Sun than Mars, you see Earth has phases just like the Moon. In fact they both have the same phase (the Sun is off to the right in this image). If you look at the full size image, you can make out the cost of South America. It also shows the relative sizes of the Earth and Moon nicely.Sometimes we just need to see Earth from a different perspective.
Reprinted with permission from the Half-Astrophysicist Blog.