There is news that the upcoming Fedora 16 will be shipped with a brand new unstable file system called Btrfs as DEFAULT. The Btrfs is not yet stable according to wikipedia.
But surely it will be a star to the crown of F16 like Gnome 3 of F15. The new FS is rich in features.
No need to worry about the current data in ext3 or ext4 filesystem. They can be easily mounted as Btrfs with write support.
The Btrfs is revolutionary because most of the disk management operations can be done online ie you dont need to unmount a partition to perform a resize or creation of new partition.