Btrfs devel - Mar 2009 - How do I boot a system root on a multidevice BTRFS subvol?

Hello,

recently I installed a distribution I want to look at via chroot into a
btrfs subvolume on a multidevice btrfs.

Now I tried to boot this, the kernel command line in GRUB looks as
follows so far:

/kernel-2.6.29-rc7-git3 root=/dev/sdb7 rootfstype=btrfs
rootflags=device=/dev/sda1,subvol=archroot

This gives "VFS: cannot find sdb7 or bad block", like it would when
trying to mount via fstab without -o device=foo

The fstab line for / looks like:
/dev/sdb7  /  btrfs  device=/dev/sda1,subvol=archroot  1 1

Is this possible already, or am I doing something wrong?

Many thanks in advance!

Thomas