I'm pleased with all the stuff people will make with Tyvek; my cousing earned beer money making wallets out of the stuff. And as a certified cheapskate, I can see the attraction of making useful gear out of other people's trash.
But I can't really see why I would work with Tyvek. Its not really waterproof—though it's a great LW single layer rain gear, as it is breathable. It will work as a tarp or ground sheet, but so will poly plastic (which is far more waterproof).
Its not the material itself, however, that argues against it. Nonwovens are probably the LW fabric future (witness Cuben Fiber). But to make a quality thing out of Tyvek you have to do just as much work as to sew something out of durable, and perhaps even more lightweight woven materials. 1.1oz nylon can be had for ridiculously cheap, and if you value the hours you spend at a sewing machine or taping things together, you'll get a better, more durable product than by using tyvek.
I'd be cautious about using any old home wrap. I started my PCT adventure in 1998 with a 10' square of the stuff I got from my uncle, a carpenter. I assumed it was tyvek, but in fact it was a tightly woven poly sheet with micro perforations for vapor permeability. I noticed this while sleeping under it in good weather, thankfully, then trashed it at the next stop and went the rest of the way under blue woven tarp, 8'x10'.
