Tom: If you have a tent or tarp overhead, dew won't be a problem. And if you pitch the tarp/tent so that you get good air circulation, condensation on the inside surfaces won't be a problem either. For a ground sheet, my preference is a 4X8 sheet of tyvek. It's light, water resistant, and big enough to fit a pad/bag and some gear. I've had water running under the tyvek and stayed dry. I also bring a large trash bag to put my pack on if the ground is wet. Works for me but I'd like to hear what Kevin uses too.
