This is because GRUB doesn't yet have its own LUKS support, so can't detect that the filesystem is btrfs and doesn't know to add rootflags=subvol=@.
This isn't likely to be fixed for natty.
This is because GRUB doesn't yet have its own LUKS support, so can't detect that the filesystem is btrfs and doesn't know to add rootflags=subvol=@.
This isn't likely to be fixed for natty.