With all the ongoing hype of Windows Vista and everyone yearning for the completely new Windows 7 wouldn't you think that Microsoft would try to improve on what they have?
According to Mary-Jo of All About Microsoft there is a completely new operating system in the works, so revolutionary that it doesn't even have the Windows moniker. (yet)
There really isn't much information about about the operating system they call 'Midori' due to it being in a very early incubation stage.
However we do know that it is it being based on their Singularity platform which is a a microkernel-based operating system. Expect this to be far into the future, post Windows 7. (whenever that happens)