Regarding the installation of your own OS... it would certainly be within Google's deviousness bracket to install a firmware hack (pre bootloader) that scans the partition table of the HDD and refuse to boot if it detects something other than Chrome OS on there.

Circumventing such a thing would take more ingenuity than the average netbook user would possess. However it is (as you implied) unlikely that such a provision would be present on the netbooks.

Who knows though?