The much lauded WinFS that Microsoft had originally planned for release with Windows Vista has met yet another setback. Delayed multiple times due to development and deployment issues, it was thought for a long time that the filesystem would not come by default with Windows, but be available later via an NTFS to WinFS conversion, possibly available for both Vista and XP. Now it appears that it it may not come out for many years, possibly past 2010. A change to the way they will deploy the file system, possibly due to integration issues, is the cause:

These changes do mean that we are not pursuing a separate delivery of WinFS, including the previously planned Beta 2 release. With most of our effort now working towards productizing mature aspects of the WinFS project into SQL and ADO.NET, we do not need to deliver a separate WinFS offering.
They do mention that Vista and future MS products will continue to "adapt", meaning that down the road WinFS support could eventually be added. The changes aren't completely clear however, and plans could be changed. The old model of WinFS is largely unchanged, but don't expect to see it anytime soon.