Comcast has finally lifted the block that prevented access.
The folks over at Comcast have an issue with HBO Go. Since streaming content from HBO is already available through Xfinity, it seems that Comcast doesn't see a reason to let their customers access it anywhere else. The app has been blocked on a few different devices including Roku players, Samsung TVs and, more recently, the Xbox 360.
Today the HBO Go app will finally be made available to Comcast subscribers that connect with an Xbox 360. If you're trying to connect through a Samsung TV or Roku player though, you'll still be blocked. It's not clear why one was cleared up but the others weren't.