Standalone hbo will probably not happen
Hbo is probably looking to sell their service through an existing service. They probably don't want to get lost in the black hole of the Internet when cable disappears. My guess is someone like amazon, apple, google or Microsoft offers them some dollars to sell their services as an add on to their existing services. My bet is on amazon. Netflix is best set up to do it, but they don't seem to show any interest in add on programming.
Amazon super prime with ESPn and hbo for 20 a month would probably get a lot of people cutting their cords and canceling Netflix.
They could provide services through a CDN or Navisite, that isn't the problem.
The problem is: DRM and direct to consumer.
1. DRM -> All those devices. That takes a lot of development $$$ and support which leads me to (2)
2. Channel - HBO since it's inception has followed the channel model. Going from someone handling your sales and customer service to a direct model puts you in competition with your channel (Comcast, TW, VZ, etc) and also now adds cost to support both a direct and a channel model. I see it unlikely until HTML5 normalizes that they go direct in the US, and their subscriber growth in the channel is impressive probably due to GO with offseason stickiness. So if they GTM it will probably be with a neutral channel partner or use an existing one to get synergies. So Netflix/Amazon are seen as competitive, so I don't see that happening FIRST.
3. TWC is starting to show it's willing to expand on endpoints, so maybe the xbox promo is a view into what's coming. The lack of codec support on XBox is a killer for hidef buffs tho. I use my Roku if I want to watch anything in hidef.