I think it’s a testbed frame — thicker, with visible (un-Apple-like) seams, meant to fit in 3GS cases so as to disguise units out in the wild. It’s hard to tell from the photos. But I think it is the real deal in terms of the internals and display being next-gen iPhone hardware.
This is exactly what I thought: that it could be a test mule, not the enclosure designed for customers. But I couldn’t figure out why Apple would have test mules of the next-generation iPhone out in the open. Well, here it is: if that phone does have a 960x640 screen, it makes sense that they would need to test it a lot out of the lab for compatibility with existing apps.
But John Gruber seems to believe this design could have something to do with the final product — that it would be a rough draft — and I still disagree with that. I think that thing’s form is not indicative of anything, and just motivated by the need to easily assemble technical prototypes.