Leaked Pro Mac Photo Summary
A MacRumors forum user posted a copy of this PDF (removed per Apple) on Friday, Jul 19, 2002 claiming to be a diagram of an upcoming PowerMac G4 design along with some details.
While trying to verify the information in the PDF, the author posted a thread with the PDF, while MacBidouille posted actual photos which corresponded with the images found in the PDF.
Meanwhile, a MacRumors source confirmed the authenticity of the enclosure, and Apple Legal requested the removal of these images hosted on MacRumors.
Note that the motherboard bears a resemblance to a leaked motherboard that appeared on eBay. Photos of that motherboard have also been removed at Apple's request.
Update: The case appears to match various specs that were rumored to be on Prototype cases including white speaker, front mounted headphone jack, 4x512MB RAM, new motherboard, bays for two optical drives.