Any app, but not on any device as I found out when I tried to activate HBO GO on a Roku. Apparently if Comcast is your ISP, you cannot use HBO GO on your Roku (or PS3 for that matter).
EDIT: You can see what devices are supported here: http://www.hbogo.com/activate/
Its important to note that there is a difference between what they restrict in the way of internet traffic, and what they sponsor in terms of allowing access to specific content over the internet.
The services that you log into via your Comcast account (I use HBO Go, HGTV, NBC Live Extra...) are provided by an agreement between the content provider and the carrier. They aren't stopping you from using HBO Go on a Roku with their internet service, and your friend with FiOS can hook his Roku up and watch shows at your house over your Xfinity connection.
I guess it's more that you cannot activate HBO Go on a Roku using your Comcast credentials as Comcast is not an option in that configuration. Not that they block the traffic or something.
I agree with the distinction, still just as irritated.
I can use the HBO Android app with Comcast just fine and push to a Chromecast, but not the Roku.
It's a ridiculous situation, Comcast is intentionally blocking Roku while HBO is embracing it.