Neowin posted a tutorial about changing the Metro background on x64

The new Windows 8 Developer preview offers the first official look at Windows 8 for the general public. This early build was designed to give a high level look at the new Metro UI and because of that, omitted some simple features like the ability to change the background.

Neowin has already covered how to revert back to the standard start menu, but if you want to be able to change the background, the tutorial is posted below. We should not that this is for x64 builds of Windows 8 only, the x86 tutorial is still being developed.

  Windows 8: How to change Metro background on x64