wakusei
God-Raped-Me
I think they did already...
Did they test breaking it with all sorts of things or just bullets?
Just bullets, I believe.
Anyway, bulletproof glass works by laminating sheets of glass and more flexible sheets of polycarbonate. The brittle glass eats up a good portion of the force of the bullet allowing the more flexible polycarbonate to absorb the remaining energy.
So, if you were to apply an even enough force, I think you could break "bulletproof" glass. Realizing, of course, that you wouldn't be trying to punch through 5" glass (the thickness of the windows on the President's limousine).
That's just my, probably wrong, thinking though...