Thanks for the replies everyone. Looks like there are people selling Tyvek sheets on eBay for just this purpose: http://www.ebay.com.au/sch/Camping-H...R40&_nkw=tyvek

Maybe if I plan carefully it can be used as either an ultralight tarp, or a floor protector - which I'll use on occasion.

Stay tuned for a thread about constructing ultralight tarps out of tyvek