Over the weekend BGR posted some photos of what appears to be a white iPhone being field tested on T-Mobile. Yes T-Mobile.
The white iPhone in the photos looks virtually identical to the current iPhone 4 running off-the-shelf iOS 4. The pictured device appears to be running on the T-Mobile 3G network and has an ominous warning on the lock screen (which didn't appear to discourage the leaker.) The device is shown with several test settings and two special field-testing apps: MALogger and FieldScout.
Speculation abounds that it could be the iPhone 4S, an iPhone 4 with the same A5 processor found in the iPad that's being given to hand-selected developers for testing - but a 4S tuned for T-Mobile's 3G bands.
26 photos of the device are posted in the BGR gallery.