Chrome OS runs other OS too. Why? Because Google wants that the hardware should support rest of the OS and built that kind of notebook. It is not revealed by Google yet but many folks done it right by applying all the possible boot techniques. At Somethingaweful forum, user drasticactions post some pictures of his Chrome OS notebook installing Windows 7. Also he posted a method to flash the CR-48 with stock BIOS which can be done after removing the bottom panel of the notebook.
After installing Windows 7, you will get only 2GB of disk space. Also 3G modem doesn’t work properly.
Chromium OS site already show you the method to boot Ubuntu on Cr-48. A user having YouTube profile Damis648 has installed OS X on his chrome notebook. According to him most of the hardware is supported by Mac OS X. Check out the video.