Grub can boot off BtrFS filesystems. There is one caveat remaining (bug 759772: grub's core.img needs to be embedded in the correct part of the btrfs partition)

Grub can boot off BtrFS filesystems. There is one caveat remaining (bug 759772: grub's core.img needs to be embedded in the correct part of the btrfs partition). When using GPT (required with large disks) that part is already safely located on a 1MB GPT boot partition.