Today, a posting on the Fedora mailing list announced that BTRFS will not be the default filesystem in Fedora 16. 
When the development of Fedora 16 started, the team announced that they would use the innovative BTRFS filesystem as default choice. 
thread.gmane.org/gmane.linux.redhat.fedora.devel/152285 

At the time, that was heavily discussed in the community because the next generation Linux filesystem still has many problems. 
One of the is, that there are still no tools available to check the integrity of the filesystem. 
But the availability of such a tool at the time of the first Alpah release was a requirement for BTRFS to become default in Fedora 16. 

Originally scheduled for May already, a tool for that should appear very soon. 
With that in mind, it’s very probably that BTRFS will be the default filesystem in Fedora 17. 

In both, Ubuntu and Fedora, BTRFS can be chosen as filesystem manually.