To Liath's point, Bear Grylls (I know, I know) did an episode of Man Vs Wild called "Norway, Edge of Survival" that had a interesting experiment in hypothermia hidden in the middle it. They simulated wind and rain at night, and his goal was a make a shelter and get a fire going. (These are something our emergency kits should help us with, IMO.) He aborted the mission when the hypothermia risk got too high, and he couldn't get a fire. He made a smart observation about the importance of waterproof clothing.

That episode has stuck with me since it originally aired a few years ago, and reinforces my personal belief that a rain poncho is an important addition to any pocket kit, or GHB/BOB/INCH/whatever, at least in this neck of the woods. Combined with hikermor's look for natural shelter approach, it's a pretty good shelter solution for an emergency bivvy. (The links on YouTube suck for isolating just this segment, and other than the river segment, I shake my head at most of the rest of the episode, so i'm passing on including links.)