This is something that should have been natively implemented in Windows *a long time ago*. Ultramon has been doing this for ages, even if by using the trick of 'stitching' different wallpapers in memory.
Windows 8 seems to have some nice new features. Unfortunately Metro, and especially Microsoft's stubbornness in forcing users into it and not allowing them to turn it off and revert to the classic Windows UI, is going to ensure Windows 8 is DOA.
Microsoft needs to be taught a lesson, understand that this is the wrong direction to go. The only way we, as consumers, can do that is by keeping our wallets shut.