I want to run a plain Ubuntu install, but the BIOS does not easily allow for booting into USB drives. I currently have the Chromebook in developer mode and I found a USB command on the chromium developer info website, but I not sure how to enter the command in.
I know this reply came a bit late, but if you're still looking to replace ChromeOS completely with plain Ubuntu, have a look at this article: http://johnlewis.ie/pre-built-coreboot-firmware-for-chromebooks/
Warning, this involves hardware hacking and will most certainly void your warranty. I've detailed my personal experience here: http://askubuntu.com/a/361561/97517