Brought up on the Fedora development list are the plans for Btrfs in Fedora, which provides a target of Fedora 16 when EXT4 will be replaced by Btrfs as the default Linux file-system on new installations.
Fedora was one of the first distributions to deploy support for installing the Linux distribution to a Btrfs root file-system, while the Moblin/MeeGo camp has already turned to it as the default file-system, and now Fedora may finally have everything ready and are comfortable with the state of this Oracle-sponsored file-system.
With the intended Fedora deployment of Btrfs, they will also switch from using LVM as the disk's volume manager to instead relying upon the volume management abilities built into Fedora itself. Fedora / Red Hat developers have previously already worked on taking advantage of other Btrfs features, like snapshots, to provide system roll-back support by creating a copy-on-write snapshot prior to each yum/RPM package transaction.
In order to meet the Fedora 16 Btrfs target, Btrfs support needs to be merged into Fedora's GRUB package (or a separate non-Btrfs /boot file-system must be used), Red Hat's Anaconda must be hooked in to properly utilize Btrfs and its volume management features, and the Fedora LiveCD must be re-worked a bit to handle Btrfs. Additionally, the fsck support for Btrfs must be completed. Oracle's Chris Mason is nearly (circa 90%) complete with this task.
Before Fedora 16 arrives we still have Fedora 15 to worry about, which will continue to offer Btrfs as a file-system option while EXT4 remains the default.
Canonical is also evaluating when to switch to the Btrfs file-system and we may see that move similarly happen for Ubuntu 11.10.