I haven't read all the way through this thread so my comments may have been covered. A woodsman who had been in that situation a few times gave me some pretty good advive. He said to find the thickest or most shelteres area you can (wind protectin and an evergreen canopy) and stay on your feet.
It sounds odd but you can regulate your temperature much better on your feet. Running in place, some squats, etc. will raise your temperature. If you get stuck without a tarp, bag and stove, keep it in mind. Your body will want to lag down but it will sap your heat.
