I've just tried putting usr in a subvol. Installation proceeds normally, no errors, but I'm dropped to a dracut prompt which indicates mount of /usr failed. dmesg follows:


[   14.025215] systemd[1]: Starting dracut initqueue hook...
[   14.077890] Btrfs loaded
[   14.129987] device label fedora_f18v devid 1 transid 31 /dev/sda1
[   14.313607] device label fedora_f18v devid 1 transid 31 /dev/sda1
[   14.446523] systemd[1]: Started Show Plymouth Boot Screen.
[  OK  [[   14.533662] btrfs: disk space caching is enabled
0m] Started Show Plymouth Boot Screen.
[   14.534404] systemd[1]: Started Dispatch Password Requests to Console Directory Watch.
[   14.534886] systemd[1]: Starting Forward Password Requests to Plymouth Directory Watch.
[   14.535001] systemd[1]: Started Forward Password Requests to Plymouth Directory Watch.
[   14.535067] systemd[1]: Starting Basic System.
[  OK  ] Reached target Basic System.[   15.053733] systemd[1]: Reached target Basic System.
[   15.094806] device label fedora_f18v devid 1 transid 31 /dev/sda1
[   15.251736] btrfs: disk space caching is enabled
[   15.430129] systemd[1]: Started dracut initqueue hook.
[   15.574280] systemd[1]: Starting dracut pre-pivot and cleanup hook...
[   15.735181] device label fedora_f18v devid 1 transid 31 /dev/sda1
[   15.876950] btrfs: 'root' is not a valid subvolume
dracut-pre-pivot[228]: Warning: Mounting /usr to /sysroot/usr failed
dracut-pre-pivot[228]: Warning: *** Dropping you to a shell; the system will continue
dracut-pre-pivot[228]: Warning: *** when you leave the shell.
dracut-pre-pivot[228]: Warning:
[  OK  ] Started Show Plymouth Boot Screen.
[  OK  ] Reached target Basic System.
dracut-pre-pivot[228]: Warning: Mounting /usr to /sysroot/usr failed
dracut-pre-pivot[228]: Warning: *** Dropping you to a shell; the system will continue
dracut-pre-pivot[228]: Warning: *** when you leave the shell.
dracut-pre-pivot[228]: Warning:
[   16.314849] systemd[1]: Starting Emergency Shell…


After a bit of snooping I found the dracut fstab suspicious:


dracut:/# cat /etc/fstab
/dev/disk/by-uuid/64383cfe-c31d-4d25-97c4-4e6b7e788b26 /sysroot btrfs subvol=root,ro 0
/dev/disk/by-uuid/64383cfe-c31d-4d25-97c4-4e6b7e788b26 /sysroot/usr btrfs subvol=usr,subvol=root,ro 1 2


Two mount options, mounting two subvols to the same mount point? Seems problematic.  But even when I try what I think is correct from a dracut prompt:


dracut:/# mount -o subvol=usr /dev/sda1 /sysroot/usr
mount: /dev/sda1 is already mounted or /sysroot/usr busy
        /dev/sda1 is already mounted on /sysroot


The only sda device mounted at this time is the root subvol to /sysroot. So the additional question is if dracut maybe has a problem with multiple physical devices being mounted more than once, which is what happens when mounting multiple subvols? This is dracut-024-17.git20121220.fc18.

Seems like maybe two bugs?

/dev/disk/by-uuid/64383cfe-c31d-4d25-97c4-4e6b7e788b26 /sysroot/usr btrfs subvol=usr,subvol=root,ro 1 2

I'd say that the problem is definitely with this line having two 
subvolumes listed.
Maybe you should boot from a live CD, mount your subvolid=1 and check 
out what subvolumes you really have in there. And then re-write fstab 
manually.
If you have /sysroot subvolume, you can create /sysroot/usr as a child 
subvolume and you won't even need then to specify /sysroot/usr in your 
fstab because it will be mounted automatically when the parent subvolume 
gets mounted.