“No, it’s not. We had a lot of internal discussion on this, and have concluded that it’s more Android than ChromeOS. To be specific, it’s actually a modified Google TV release, but with all of the Bionic / Dalvik stripped out and replaced with a single binary for Chromecast”
Chromecast made apps should be coming to the platform too. HBO Go, Pandora, Songza and many more are said to be readying their launch on this device, so although functionality is still a little bit limited right now it will be getting much better in the coming weeks and months.
If you actually managed to get your hands on the Chromecast device, which is very much in demand at the moment, you are able to root it but do so at your own risk of course. The Chromecast is a device in its infancy right now so you wont be able to install apk files on it at the moment but best believe more functionality is coming very soon.