Time to boot back into Chrome OS on my Cr-48!
"The latest Chrome OS release is only available for Samsung and Acer Chromebooks as Cr-48 Chromebooks will skip Chrome 19."
from http://googlesystem.blogspot.jp/2012/04/new-window-manager-f...
From the comments: http://googlechromereleases.blogspot.com/2012/04/dev-channel...