The space shuttle has tiles all over the bottom to keep the heat out on re-entry. Between the tiles is some filler, something to plug the cracks as it were. Occasionally, some of the filler comes out a bit. Apparently it happens fairly often, and has never caused a problem.

All the pictures I’ve seen of the "damage" have been really grainy and nasty on TV. Today I looked at the "Astronomy Picture Of The Day", and it was the shuttle, with a great shot of the underside. They don’t mention the damage, so I’m not sure if they realized they got such a great shot.
If you click here or the image above, you can see my version full size.