I use the polycryo ground sheets from Gossamer Gear under my sleeping pad in my tarp tent. It is 40x96", weighs 1.5oz and packs down to nothing. Durability might be an issue, but they only run $3 each. I only have 3 nights on mine so far, but no holes or tears yet.
http://www.gossamergear.com/cgi-bin/gossamergear/polycryo_ground_cloth.html
