There's an eighteen ounce tarptent over at tarptent.com you should look at before you start - also read reviews of it at backpackinglight.com if you really want to know how it performs.

I am thinking on getting one but only because I will be using it in the Sierras, where rain doesn't (typically) go for weeks on end - and even if it did, I will be on SAR ops and not staying in one place for more than a few hours. Tyvek eventually wets through in heavy rain.

Tyvek seems pretty durable and gets softer and less crinkly after a few washings. Used to have a groundsheet made out of it but sold it with the tent.