Thank you for reporting this issue (and fixing it).
I've pulled your fix into the "upstream" location at https://code.launchpad.net/~cloud-init-dev/ubuntu/natty/cloud-initramfs-tools/natty . I just pushed that right now.
I've also pushed a build to my ppa at https://launchpad.net/~smoser/+archive/ppa .
Could you test that and report back?
It also has a change to rescuevol that should not affect growroot at all.