I agree with many of the "manners" sentiments. As much as I am a gamer, there is a time and place for everything.
Engrossing yourself in something, whether it's putting headphones on, talking on a phone, or playing a video game, is a detachment from social interaction with those in your vicinty. This is fine in situations where you aren't expecting or desiring social interaction (sitting alone in a library, riding on the bus), but in many social situations it's unexpected and rude. Sitting at the table is a great example, as is waiting to be served. Having worked in fast food, there's nothing more frustrating than trying to serve somebody who won't hang up their phone -- you can't have two conversations at once, so by continuing to pay attention to the phone, you are implying that the phone is more important, and that's the very definition of rude.
On the other hand, much of what we consider "rude" is due to traditional social norms and these are very fluid things. At one time, public cell phone usage was a disconcerting thing; nowadays it's commonplace for somebody to answer their cell phone without going to find privacy. Even the limits of what is acceptable in this situation are continually changing. As with all technology, our ability to adjust our behaviours and expectations is quickly outpaced by new innovations. It will take us time to adjust. In the mean time, we should be aware and sensitive to the expectations of others around us.