what will be the unique "selling" proposition?
I think atm most people don't need or have an advantage running a web-only OS like Google's Chrome OS. They (we) enjoy the huge choice we have: pick a distro (or more), install it, customize it ...
If one like to create something web-only like Chrome OS he will need a huuuuge infrastructure of servers. Who apart from Google will be able and interested to set it up and to pay for it?
btw: there's ChromiumOS
if you don't like to buy a new machine.