DirectX 11 will support both Vista and Windows 7 and therefore can and will be launched before new Windows. This happens simply because graphics guys and the gaming industry want to get things to the next level and some elements of DirectX 11 will get things such as tessellation to the next level.
As you're probably guessing, DirectX 11 graphics hardware comes around the same time, late Q3 or Q4 2009.
DirectX 11 will be released before Windows 7
DirectX 11 will be released before Windows 7 
