I just read an extensive and interesting article in Bicycling magazine about advances (or the lack of advances) in helmets. It's not been posted to their site yet, or I would link it here.

Basically, what they said was that helmets were advancing under the Snell and ANSI standards as the standards were updated as technology and the understanding of injuries got better. Once the CSPC (Consumer Product Safety Commission) got involved with a standard, the manufacturers only need to meet the CSPC standard in order to be able to sell their helmet as meeting a known standard of protection. The CSPC standard is not being updated as the others are, but nobody cares as long as the can put some kind of certification sticker on the helmet.

Better technology is available at a higher price now, but manufacturers see no marketing advantage. They are reluctant to advertise the better protection of new helmets, due to fear of lawsuits if/when it fails to provide 100% protection from injury.

The current helmet designs protect some from catastrophic trauma, but not much for a concussion at lower impact levels. The issue is how the brain rotates under impact, causing trauma and current designs don't provide that sort of protection. There are helmets just now hitting the market but they are higher priced and due to the aforementioned legal fears, they won't advertise them with clear language.

I'll see if I can find some information elsewhere and provide a link.

Added: Found a similar article from Popular Science: http://issuu.com/mipshelmet/docs/popsci_mips_2012_2/9