Chromecast has been rooted by the GTV Hacker crew, who discovered the Chromecast runs a mix of both Android and Chrome. Google has said that Chromecast runs ChromeOS, but when the Chromecast is powered up it says "Chrome", but the GTV Hacker crew say otherwise.
Once the Chromecast is rooted, the GTV Hacker crew say what you see is closer to Android and Google TV than it is to Chrome. At the moment there's no way to run apps on Chromecast, but the crew didn't dismiss the idea of putting an Android port on Chromecast. You might want to get in quick, because Chromecast sold out online within 36 hours, and the Mountain View-based giant have already pulled their three-month Netflix promotion because of "overwhelming demand".