Canonical's Scott James Remnant (Ubuntu's Development Manager) has published a blog post entitled Btrfs by default in Maverick? In that post Scott describes, "We are going to be doing the work to have btrfs as an installation option, and we have not ruled out making it the default."
However, five steps first need to be reached for Btrfs to become the default Ubuntu file-system. Btrfs needs to not be marked as an "experimental" file-system within its kernel (this is being done in the Linux 2.6.35 kernel for which Ubuntu 10.10 will be based), support for Btrfs has to be added to the GRUB2 boot-loader and to Ubuntu's installer, testing of Btrfs by default during the alpha releases needs to go smoothly, upstream Btrfs developers (such as Oracle's Chris Mason) needs to be happy with the idea of Ubuntu picking up Btrfs by default, and Canonical must be happy.
If this happens for Ubuntu 10.10 that would certainly be surprising and would likely make the Maverick Meerkat one of the first Linux distributions using this next-generation file-system by default -- MeeGo's already using Btrfs. If all the pieces don't come together in time, we'll likely just see Btrfs become an alternative file-system option in Ubuntu 10.10 and then see the move to it being the default in Ubuntu 11.04 or 11.10 in the next year.
For more information on the Btrfs file-system, see our various benchmarks and articles.