As planned, we took advantage of being at the Apple Expo to perform a live test. The goal was to evaluate the resistance of the glass plate sitting in front of the LCD panel of an iMac 24" Alu:

The glass plate turned out to be stronger than expected, and could resist to quite substantial shock despite its thickness.

However, it is NOT a "securite" type of glass, and when it broke, we obtained sharp glass pieces. Nevertheless, your iMac 24" will have to fall, or you'll have to intentionally hammer it to potentially have the glass plate to break in pieces like that.