My guess is it's for the new hardware. The new Minis and Airs have Bluetooth 4.0, mid-range Mini has a new graphics card, and the Airs have a different Thunderbolt controller. Who knows what else has been tweaked in there. But all other previous hardware don't have these things. This is typical Apple (aside from the 2 different new builds, but I suppose they don't usually launch 2 new hardware models at the same time). 10.7.1 will likely unite them all again. The new builds will run on the older hardware, GM build won't run on the new hardware.