Josef Back said outright in a mailing list message, "BTRFS WILL NOT BE THE DEFAULT FILE SYSTEM FOR F16."
This follows many users and developers wondering whether or not Fedora 16 would change to Btrfs from EXT4. The reason for delaying the switch is that the Btrfs engineering requirements haven't been met. In particular, there still isn't a suitable tool for testing and repairing Btrfs file-systems. Such a Btrfs utility is expected soon, but the deadline for having one ready was Fedora 16 Alpha. As a result, Btrfs isn't ready until at least Fedora 17.
Btrfs is a very nice file-system and has numerous advantages over EXT4 (e.g. SSD optimizations, Gzip/Zlib compression, copy-on-write snapshots, etc), but for now you will need to manually opt to using Btrfs when installing Fedora. In the Ubuntu camp, they will not be moving to Btrfs until at least Ubuntu 12.10 (October 2012), but there too it's currently a non-default install time option.