I'm going to go even farther out on my limb on this, and say that, unequivocally, helmets sold in the US have
NOT deteriorated in construction or design, nor has the relative safety provided decreased.
I had to wonder about Bell's reported change from Snell compliance reported in this thread. I checked a couple of helmets I have, both Giros. They both have a sticker that says words to the effect of "CPSC compliant". This got me wondering more. When I was doing my research, Bell was in the forefront of supporting ANSI standards, and that was their corporate culture. To have dropped that would have meant an extreme turnabout in corporate culture.
So I did a quick google for the CPSC bicycle helmet standard. The first page, top five hits got me gold. The ANSI standard was passed in 1984, and has morphed, with some evolution, into the CPSC standard. The evolution involved did not water down the ANSI standard.
References:
http://www.cpsc.gov/cpscpub/prerel/prhtml98/98062.html
http://www.helmets.org/, particularly
http://www.helmets.org/testbycost.htm, also
http://www.bhsi.org/standard.htm#ANSI
The Snell Foundation apparently now has two standards. B-90 and B-95. All bicycle helmets sold in the US must meet the CPSC standard. It is required. Snell's B-90 is apparently identical to the current CPSC standard, B-95 is slightly tougher.
Unless the Bell culture has changed, any Bell bicycle helmet you purchased in the past, after 1984, would have passed the ANSI standard, and later the CPSC standard. Since the Snell is slightly tougher, if they also meet the Snell B-95, then they might be a little more absorptive of energy. As I said in my first post, in the US it is the ANSI standard that has driven bicycle helmet design, not the Snell. Just a market thing - ANSI had consumer recognition, Snell didn't.
It costs the mfr money to get the Snell certs - it ain't free. So, if it doesn't market well, why bother? The helmets on the US market today are passing the same safety testing they did 25 years ago.
Go to the market and buy the helmet you like. There hasn't been a deterioration in their design.
Cheers;
hiero