Pandora recently recently received Chromecast support, as did Hulu Plus, leaving HBO Go as the lone man out. According to the Google support page for Chromecast, support could be coming very soon, however. HBO Go is shown next to Pandora and Hulu Plus as a “new” app to Chromecast in the compatible apps and services section.
Someone probably slipped up and put this on the site prematurely, as there’s no description for HBO Go like there is for the other supported services. It’s still exciting to know that we should see support for the streaming service any day for Google’s insanely enticing $35 streaming dongle.