Didn't you make one of Henry Shire's TarpTents out of Tyvek that had a problem with leaking? I know that John has been able to use Tyvek for his rain gear successfully, but other posters have had problems with it. I purchase a Tyvek shirt and pants last fall and gave them a test in the my shower (probably not a great test). I seam sealed them and then the next day stepped into the shower. With very light spray they seemed fine, but in anything beyond that the material got overwhelmed and wetted out after a few minutes. It seemed to me that the Tyvek performed more like a DWR coating which is great for light rain and brief showers, but not what I would pick for a shelter.
P.S. The shirt and pants make a pretty good wind suit and are certainly light and cheap so I have used them, I just don't trust them for rain.