I have been wearing Vibram Five Fingers for several months now. I have used the Classics and am currently spending significant time hiking in a pair of Flows. One thing I have noticed is that in cold condition, my toes go numb and this numbness can progress to the rest of the foot quite quickly.
I have alleviated much of this discomfort by wearing two pairs of tracksuit bottoms (Ron Hills if you must know), in a bid to keep the lower leg warm. This has worked quite well and meant that only my toes remain cold - the rest of the foot warming up in all but the coldest weather.
I am currently in the process of experimenting with 'leg warmers' made from the ends of an old wetsuit in a bid to keep the lower foot warm (think of a 'gauntlet' for the lower leg). I considered an alternative approach to try and make a full Five Finger neoprene stocking (like a flexible/neoprene wellington) - although with the disadvantage that you'd be unable to remove the upper section in hot and dry weather.
It occured to me on my last hike in cold and wet conditions that the Flows were working on the same principle as a wet suit and that a pair of neoprene gators/leg warmers/stockings might well aid in keeping the lower leg warm and so perhaps the whole foot - thus being a useful addition to the range of Five Finger accessories.
Time to get stitching and cutting.