That's true in principle, but current implementations are often poor. On PS3, for example, you either have notifications on or off; there's no settings for different activities. That forces the user to go and change a setting each time they change activity, or, more likely, forget to do that as it's not a usual activity when using a CE device and then be intruded watching a movie. This isn't a problem for me on PS3 as I don't auto-sign into PSN. I only sign on when I want to play online. This could change next-gen with forced network connections, at which point the management has to be much better than this gen to provide a good user experience. The defaults should also be sensible instead of requiring users to micromanage their settings, which plenty will have trouble with. By default, it makes sense not to have notifications pop up in a movie as that's an obvious intrusion, whereas when playing online, notifications should be on by default.
The devil is in the details, I guess.