GTV Hacker has posted some news regarding advances in hacking the Chromecast in the form of a root exploit. As described by the original post here, "This will allow researchers to better investigate the environment as well as give developers a chance to build and test software on their Chromecasts. For the normal user this release will probably be of no use, for the rest of the community this is just the first step in opening up what has just been a mysterious stick up to this point."
The site has also offered up it's interpretation of the exact operating system the device is running, saying that "it's more Android than Chrome OS." The software contains a lot of recycled code for Google TV, but has had the necessary goodies to actually install and run APKs stripped out.