Chromecast is Android-based, already rooted
More accurately, the Chromecast is just what we said it was from a feature standpoint: it's an extremely stripped down Google TV. That means that it is basically Android with the Chromecast binary running. According to GTV Hacker, "the bootloader, kernel, init scripts, binaries, are all from the Google TV", but you won't be able to install APKs.
Of course, the direct result of learning that the device is more Android than Chrome is that the device can be rooted. The only trouble is that it is unclear how useful that root method will be. Chromecast is a very limited device from a hardware standpoint, but there is hope that it could become a full Google TV with a little bit of prodding. We'll let the dev community look into it, and we'll let you know if anything interesting comes from it.
source: GTV Hacker