This is my first post here so take this advice knowing that. I had been thinking of doing the same thing as you. I was looking for a small, lightweight and inexpensive laptop to use as a pen test platform. After my research the answer seems to be no. There is an Ubuntu version that has been ported to work with the Chromebooks called Chrubuntu or something similar but I haven't seen anyone get Backtrack working on a the Chromebooks. The Chrome OS uses a custom boot loader and GRUB does not work right. While it seems likely that you could install Chrubuntu and add packages you need but that would be like compiling your own distribution. I am not interested in doing that.
What I ended up doing instead is buying a model similar to the Chromebooks but with a Windows license for only $100 more than the Chromebook and I have a much more useful piece of hardware. Windows 8 is a nightmare but I have been able to use Backtrack 5r3 in a virtual machine. That leads me to my next point. It seems that VirtualBox does not support USB 3.0 and as such I haven't been able to use my USB wifi adapter. The Alfa adapter works on the pc and can be used by the virtual machine but only sees an Ethernet connection and no wireless adapters. Again, this appears to be a problem with VirtualBox not recognizing USB 3.0. I am going to be trying VMware to see if it works better. I hope this helps your decision making.