We should add support for resizing btrfs to virt-resize. This will be particularly important since F16 installs may default to btrfs.
I added support for resizing btrfs upstream: http://git.annexia.org/?p=libguestfs.git;a=commitdiff;h=f2d816573ffba9c5a2022ac5b3b6514eaf02d0f7