When I visited GFW High School in January, the principal mentioned that although some iPads had been damaged, not a single unit experienced broken glass. I believed him, but it seemed incredible. 9 to 5 Mac posted this video from ifixyouri featuring a glass strength test on the iPad 1 and iPad 2. (Note: turn down your speakers)
Now I know why none of the iPads experienced broken glass. The aluminum back and internal components are more fragile than the glass (Another note: I did develop a small scratch on the bottom of my iPad 1).
Here are some examples of how you can deliberately break the glass of an iPad (not recommended):