According to The New York Times, in order to attain the SNELL M2005 certification, a helmet needs to be able to withstand that impact of a small steel sphere, twice, with the headform inside the helmet experiencing less than 300 times the force of gravity (300 g's). That's a huge amount of force, and it has led to SNELL M2005 helmets being so exceptionally rigid that some head injury researchers believe riders are suffering brain damage because of it. Also, SNELL doesn't use headforms of different weights, so riders with small heads can especially disadvantaged because of the disproportionate forces they'll encounter in such a firm helmet.
Some helmet companies and doctors and the U.S. and European helmet certification authorities believe that the SNELL M2005 test is obsolete, and that helmets should be "softer, softer, softer." SNELL has admitted to the need to update its test, and will be unveiling the M2010 standard shortly. The problem is that the M2005 standard helmets will continue to be sold in America until 2012. They won't be able to be sold in Europe until SNELL re-engineers its test to allow for more forgiving headgear. For you riders out there, do your homework, ask around, and make sure to buy the helmet that's right for you. A cranium is a terrible thing to waste.
[Source: The New York Times | Image: SNELL Memorial Foundation]