It's not clear to me, why you would need to swap boards in order to run another OS on the ChromiumPC.
That is pretty weird, yeah. At a guess, I'd say it has the OS as firmware to make the bootup really fast, and you'd be replacing that firmware board with a BIOS board.
ChromeOS netbooks include an actual ROM (not eeprom) that starts a boot chain that cryptographically evaluates each next step for signed google code. It does have a dev mode feature where unsigned code can be run, but it makes it a slow and somewhat annoying boot so that the user understands they aren't running a blessed chromeOS. If you really wanted to run something other than chromeOS all the time, you'd likely not want that feature.
Hmm, so like some recent Android devices from HTC & Motorola, ChromeOS devices use a signed bootloader. Got it.