Legit Reviews came across a video that shows how though the Nokia Lumia 900's glass is:
Well, if this video from Nokia's hardware validation lab is anything to go by, the Nokia Lumia 900 is very tough indeed. The video below shows one being torture tested by having a nail hammered into its glass face and then the phone itself being used to hammer a nail into a piece of wood! Incredibly, it survives this without a scratch and is still fully functional at the end of it. Of course, it must be remembered that the shocks go right through the phone, so the circuit board, battery and case must all be made to be very tough as well. Note that trying such a harsh test on your own phone is not recommended.