I wear the 661 FBC, good stuff I wore it all the way up my local trail and it wasn't too hot. bearing in mind that it wasnt a hot day. Never crashed with it so far. I was gonna get the troy lee D2 but the fit was a little odd plus it was 100 dollars extra. The DOT helmets are designed to withstand really hard impacts like with car bumbers. So they are really hard. This isn't necessarily a good thing though for mtbiking. Helmet shells are supposed to be soft enough to eat some of the force of the impact. A harder helmet will withstand harder hits but for the forces you would expect in a MTB crash they won't cushion the blow very much. Whereas a MTB specific FF helmet is going to be a little softer and is designed to specifically take the kind of hits you would normally worry about on the trails. For real DH riding it is possible that you may really want a Snell/DOT certified helmet because DH can take some really hard hits. I'm confident in my Full Bravo Carbon. I tried a couple of the DOT helmets and the added safety was at just too much of a cost in weight and comfort. Only the Fox Racing V3 is up to DOT and SnellM2000 standards and is still under 3 pounds. Under by half and ounce. If money is no object and you do want DOT, then the TLD SE is a good one to look at.
Check out the June 2005 issue of Mountain Bike Action. It should be on the shelves right now. It has an indepth articla about helmets and also has a list of all kinds of helmets and their basic specs.