There is an interesting article in the June 2005 edition of "Motorcyclist" on helmet certification standards. Based on their testing, they conclude that helmets designed to pass the Snell standard have shells so hard that they actually transfer higher G forces to the head in an impact than softer-shelled DOT-only helmets.
For example, in a head-to-head test (pun intended) between a $500 Arai Tracker GT (DOT/SNELL 2000) and a $70 Z1R ZRP-1 (DOT-only)
Z1R: min = 130 Gs, max = 176 Gs, mean = 152 Gs :?
Arai: min = 166 Gs, max = 243 Gs, mean = 201 Gs :o [/list:u] where a 200G, 2ms impact is considered "bad" for most people. Interestingly, the Snell standard allows 300 peak Gs, whereas DOT only allows 250 peak Gs. More interestingly, after testing, the shell of the Z1R actually looked better than the Arai, and its internal foam at all impact zones was still far from bottoming-out.
Just thought I'd pass this on. It may be old news to you folks, but it was alarming (and annoying) to me, especially since "Snell is best" has always been the widely-repeated mantra.
-Gavin