I thought I would add some input here as I was just talking about this on another forum tonight.
Tyvek is a wind/water barrier but is permeable to water vapour. Made up of a mono-filament membrane, it is used to keep the water vapour levels in your insulation equal to the outside level.
Ironically, tar paper in two layers around a house can do the exact same thing. I think it's more of an issue of expediency when applying the housewrap and Tyvek's durability that makes it a popular choice for builders.