You've no doubt seen one of the newer (~10 years or so) phases in motorcycle helmets; the modular, or flip-face helmet. It looks very much like a normal full-face helmet when it's closed, but the entire front, including the face shield and the chin guard, flip up out of the way. This, I suppose, is so you can eat, drink, smoke or what-ever while you're riding.

They have to be DOT approved for highway use, which they are, but do the pass the SNELL test? As of this writing, I don't know. I haven't been able to find any modular helmets that have been submitted to them for testing.

I did, however, find a comparison done by Motorcycle Cruiser, who stated that the modular helmets for the most part were "acceptable". They did a number of tests that appeared to simulate the SNELL tests. Here's what they had to say:

At the end of the day we had learned that all of these modular motorcycle helmets provide acceptable crash protection, as long as you make sure they will stay on your head by performing a roll-off test before you buy. For the motorcyclists who wants the best protection in this crowd, we suggest looking at the Nolan, Lazer or Shoei.

So if you're thinking about getting one, go check out Motorcycle Cruiser and read the entire write up and decide for yourself. If you're cruising or touring, it might be the right helmet for you. But if you're a sport rider, you may want to think twice.