An often-blurry QuickTime spy video of what appears to be an aluminum MacBook enclosure is available now on apple.pro. (If the video link isn't working, try watching it on YouTube; there are stills available here.)
The video is a series of close-up shots, none showing the thickness of the enclosure, nor the keyboard. The only features visible in the video are the new location for the power button, and a larger trackpad. Its authenticity could not be independently verified.
Thanks, Josh, for the YouTube link!