The Fedora developers have decided that Fedora 16 will not use Btrfs by default. The announcement was made by Josef Bacik, Senior Software Engineer at Red Hat.
According to proposals for Fedora 16, Btrfs will be the default filesystem used in that release. The proposal has been approved by the Fedora Engineering Steering Committee.
Canonical planned on introducing Btrfs support in the upcoming Ubuntu release, i.e. Ubuntu 10.10 Maverick Meerkat. According to a recent update in the Ubuntu Btrfs Support blueprints, Btrfs will not make it to Maverick.