Brockton Cycle Center says:
The inner foam lining of the helmet compresses, and that is what provides the majority of the protection. The inner foam lining is also what causes a helmet to 'age'. The foam itself dries out over time, and a good rule of thumb is to replace your motorcycle helmet every 5 to 7 years. Storing a helmet in an area free of significant temperature changes, excess humidity, and petrochemical fumes will help extend the life of the helmet.
Another thing to remember is to keep it out of the sunlight as well. Any polycarbonate type plastics (and plastics for the most part) deteriorate quicker in UV light. The foam is usually what goes "bad" first, but the shell itself can age and lower the proctection over time. I'd bet a helmet properly taken care of can last longer than recommended 3 year rule, but I've always accepted that as a rule of thumb because you can't tell what's really going on at the molecular level. Even without dropping a lid you can't tell if one area of the helmet has become weaker than another.
We had to do a little report for one of my engineering classes last year and I did it on helmets. There's two compaines out there that are coming up with some interesting designs. One is a spin-off company that used to do a lot of work for Nike. They're using a double semi-hemisphere air pocket design, similar to the innards of a running shoe, as a helmet liner. Supposedly from their tests (which I take with a grain of salt) there is a huge reduction in the G-forces taken to the noggin in an impact.
I'm in the camp that more expensive=better comfort, but not better safety. I've had a HJC CL-14 that I've been wearing for the past 2 1/2 years. Noisy as hell at high speeds, but comfortable. No idea what I'm going to buy next until I hit the local stealers and try some on. Hard finding a good fit with my massive head.
I'm happy to hear that Snell is rethinking their position. I remember reading that article and their immature response to the mag. Petty bickering like that pisses me off. This is about keeping people safe out there on the road, so suck up your egos and change your policies if you need to.