Only one, as far as I know
I bought an Atmos one week ago, and the store had both 05 and 06 models. As far as I could tell, the only difference was in the color schemes. I ended up buying an 05 in blue/white (Fassa Bortolo colors) since it was on clearance. It's by far the best-ventilated helmet I've ever owned, probably equal or superior to the other top end helmets out there in this regard (Giro pneumo, Bell Sweep and Ghisallo, Specialized S-1 and Decibel). Don't bother with the flashy helmets with gimicky cooling mechanisms (eg Briko Twinner, LAS Haxial) unless you're looking for a helmet to keep your head warm during cool weather! I had a Briko Twinner for two years (I have no idea why I rode with this torture device for so long) and the only thing at which it excelled was generating a never ending stream of sweat from my forehead.