Fedora was the first to support optionally installing to a Btrfs file-system and they even took it a step further by providing system rollback support using Btrfs. MeeGo though was the first major Linux distribution using Btrfs by default and Novell has also joined the Btrfs party by providing Btrfs support.
With the latest Ubuntu 10.10 packages there is now support within GRUB2 for Btrfs (but for now the /boot partition cannot be Btrfs but needs to stick with another file-system like EXT3/EXT4 for now) and the alternate installer supports installing to a root Btrfs file-system when using the manual partitioning options.
As is said on ubuntu-devel announcing this Btrfs milestone for Ubuntu, "This is still NOT RECOMMENDED FOR PRODUCTION USE and MAY EAT YOUR DATA, but we're making the option available by way of manual partitioning only so that we can experiment with btrfs more easily, contribute fixes to various tools as needed (as we've already done with grub2 in order to at least get this minimal level of support in place), and the like, and hopefully to encourage some more people to get involved in its development."
Btrfs is interesting for its snapshotting/sub-volume capabilities (like what can be done with system rollbacks and we also have a Phoronix Test Suite feature taking advantage of the Btrfs snapshots to be announced soon), competitive performance, and many other benefits. Hopefully with Ubuntu 11.04 we will find Btrfs replacing the EXT4 file-system on more computers.
You can be certain that right now we are experimenting with this new Btrfs support in Ubuntu and will report any interesting findings.