I attended a motorcycle show in Washington, DC last January. While there, I listened to a presentation by Bruce Porter from Arai about how to fit a helmet. During the talk, he provided a number of useful factoids. When talking about how to clean a face shield, he mentioned something that is very related to this thread.
One of the safety standards for helmets is that the face shield must be able to stop a bullet (22 caliber, I think) fired from close range. This test simulates a rock kicked up by another vehicle's tire at freeway speeds. The face shield can crack during the test, but must not let the bullet through.
Something to think about: this test was probably developed because enough people were getting hit by rocks and other hard objects at high speeds. Given that, and the fact that bugs make a loud thump when they hit my face shield, I pretty much always ride with the face shield down. I only put it up in stop-and-go traffic.
Here's a link to a video of Bruce Porter talking to Jay Leno about proper helmet sizing:
http://www.jaylenosgarage.com/extras/accessories/arai-helmets/