After a rather lengthy discussion of where the Mac has been, Steve Jobs got back on stage and revealed a rumor to be true. All of those "brick" rumors we'd been hearing about were confirmed. The new Apple notebooks are to be carved from a solid brick of aluminum for a lighter and stronger product.
The old manufacturing process involved an internal frame that was filled with components then bolted to a pressed baseplate. There were a ton of supporting structures inside, like under the palm rest and there was a "plastic gasket" to join the two parts together.
However, they have been looking for a better way to make notebooks, and Apple thinks they've found it. This was actually implemented on the MacBook Air. That's how it was so thin.
The MacBook Air is made by carving it from a solid piece of aluminum. A rough cut is made, then holes for the key caps are made and then its anodized. This basically means rather than taking parts and putting together, they start with a solid piece and remove bits to make it.
Apple assures they clean up and recycle what is removed. This results in a 2.5lb piece of metal that is whittled down to a quarter of a pound. This is what the new MacBooks will be made of. Stay tuned for more information as it rolls out of the Apple press event!