If you're an HBO subscriber and just can't get enough TV, then I've got good news for you! HBO has just released its official watch-it-on-the-go Android App, HBO GO. Of course, you have to be an HBO customer in order to use this app, but that's the only prerequisite as far as I can tell.
The app allows you to watch all of your favorite HBO content directly on your mobile device, including movies and HBO original content. It should work on 3G, 4G, and WiFi, but unfortunately it is limited to US customers at this time.
That's not the only downside, either. A quick look through the reviews in the Market show that it's not working properly on some phone models (like the original DROID), and that it's a pretty large file that lacks the ability to be moved to the SD card - a pretty serious omission for an app like this.
If you still want to give it a shot, you can get it for free in the Android Market.