Engadget has a video and article on Chrome OS like lightning from a USB key, with links to a torrent of the OS which can be set up to (I think) run from a USB key (with the link I mean, I know it can be set up to run off a USB key). Definitely a fun looking thing to try if you have a laptop sitting around that needs some use...

Ah, here's a site with the same download (though broken) and windows and linux install instructions.