Chinese iPhone parts suppliers are apparently now shipping the white 4G case to their purchasers. Melech518, one of our uses who runs an iPhone repair shop here in Tampa, has received the parts just this morning.
The website iPhonePortugal has got a video showing the same pieces in nice high resolution.
Melech let's us know the digitizer (glass) is much thinner than the 3G/3G[S], and the mid-frame is a solid piece of aluminum pre-cut for inner components. Check back with us when we cover WWDC live on June 7th for the keynote to put some proof or shame to all this.