If you were following the news around Apple's newly released white iPhone 4, you would already know that the white iPhone 4 is slightly thicker than it's older black brother, 0.2mm ticker to be precise. Now that's not such a big deal, however very tight fitting hard plastic cases would most likely not fit the new white iPhone 4 as of right now. Otterbox has already put up a message on each of the iPhone 4 case product pages acknowledging the difference in the from factors and saying that their Defender Series, Commuter Series and Impact Series of cases will not work with the white iPhone 4. 

Otterbox's Impact Series case is made out of 100% silicone unlike Otterbox's more tougher cases which employ both hard plastic and silicone. It's not clear why Otterbox deemed their silicone Impact Series case for the iPhone 4 to be incompatible with the white iPhone 4 at this time. The slight change in thickness of just 0.2mm in the white iPhone 4 shouldn't hinder silicone cases incompatible so if you're really in need of a tough silicone case for your brand new white iPhone 4, you should not need to worry about silicone cases.