HBO Go Support Coming
On November 13th on a Google support page for the Chromecast that lists out compatible apps and services, HBO Go was shown sitting next to Pandora and Hulu Plus as the “new” apps to the ecosystem. It has since been pulled. It was probably put up prematurely but could be an indication that the HBO Go app will soon support casting.
You do need a subscription to HBO to use the Go App, however it has almost ALL seasons and episodes of all their original programs. Even old series like The Sopranos, Curb Your Enthusiasm, Weeds and Lucky Louie to name a few. Unlike cable in demand it has every episode, so it seems like a pretty nice addition to chromecast if you already subscribe to HBO.