I have used tyvek as a ground cloth but it's not as waterproof as I'd like. I have also used Gossamergear's Polycryo ground cloth which is light, waterproof but wears out too fast. So I glued the two together and have used on the last few trips. The only drawback has been dust and crud that gets between the two layers but I just shake it out, who cares as long as my bag is clean and dry.
