I think iSkin makes the best iPhone cases. It has this dual layer design that uses plastic and silicone. I bought the iSkin Revo for the first gen iPhone and it's kept my iPhone looking like I just bought it from the store. They came out with a new iSkin for the iPhone 3G that has dual layers.
Here check it out everybody!
iSkin fuze - world's first dual-layer fused soft and hard iPhone case